Admittedly, he's hardly an amateur. Grosz Junior and his jazz guitar have appeared at festivals all over the world. He is a prolific recording artist who cut his first record in 1950, has made star appearances in movies such as Tootsie and Bullets Over Broadway and is a renowned jazz champion of original and rare works. But compared to his "old man", as he calls his father, the radical satirist George Grosz, he's not quite in the same league.
Not that he minds too much. There were distinct benefits in having parents who courted amid the wild scenes of Twenties Dadaist Berlin, whose artistic exploits included climbing up ladders and pouring buckets of water over people's heads. "Sure, we kids were overshadowed," says Marty. "But when my father was working and everything was going right, it was a 24-hour- a-day performance."
Indeed, Grosz, who died in 1959, would have been proud of his son, not least for his chosen musical style, his twang and his relentless Yankee cliches ("So, you'll be having bangers and mash for dinner, tonight, right?"). For while his work encapsulates Twenties Berlin, Americana was dear to George Grosz.
Born Georg Gross in 1893, he Americanised his name in 1916, at the height of the First World War, in which he fought. At the same time he began to play the banjo, smoke a pipe and wear clothes tailored in the American style. It was an outward symbol of his growing estrangement from the German establishment. After the war, Grosz pursued a highly politicised artistic life. Living in the heart of bohemian Berlin, he contributed to several left-wing magazines. His cartoons, paintings and drawings depicted a scorching yet passionate view of Berlin's decadence and corruption. His unblinking attack on the turmoil that was post-war Germany projected a desire for work "tough, brutal, transparent, an art that hurts. We've got enough lullabies," he wrote.
When the Reichstag was torched in 1933, Grosz found himself in the unenviable position of a Cassandra. The Fascist rule he had lobbied so vehemently against had arrived; disillusioned by the inability of either Communism or art to wreak political change, and suspecting his art had, perversely, heightened the fame of the Nazi cause, Grosz left for a teaching job at New York's Art Student's League, and took his family.
"Having been through the First World War, he could see it all happening again," says Marty Grosz. "He forecast the coming of Hitler; he had caricatured him as early as 1923. The Nazis declared him Bolshevik enemy No 1. The Brownshirts raided his studio several times. He was openly critical of the regime - but no one wanted to know."
Marty was three when George took his wife Eva and sons West. "We travelled on the Bremen, which had the Blue Riband for the fastest Atlantic crossing. When we were a day off New York," he says, "I remember watching a red sea-plane catapulting off the deck, carrying bags of airmail into Manhattan."
The family was thrown into American life in a similarly abrupt manner. "We lived in a hotel in Manhattan for a few months, then my parents moved to a house in the New York suburbs." It became a haven for fellow European intelligentsia driven away by war. "Our home was never without foreigners, people my father would vouch for, in order that they could get jobs." Publishers, professors from the University of Berlin, teachers who'd escaped from concentration camps, all came to stay. Bertolt Brecht was a frequent visitor.
The experience of exile effected Grosz deeply. Although at first he depicted Manhattan with his characteristic zest, he began to despair that his earlier work was unsuitable for the New World. "He couldn't resume his stance," explains Marty. "He was a satirist, but he refused to bite the hand that fed him. In addition, he didn't really know the language or the customs of America. All the references had changed."
While Marty and his brother Peter enthusiastically embraced their new life, their father's periods of artistic dormancy lengthened. He and his guests talked in German long into the night. "I would hear them. There was a lot of unhappiness. Guilt, at having left; but also frustration. My father would get drunk, frequently; and rail at those who had sat back and done nothing. He would say, 'Hitler rolled in, as if we hadn't been there'."
None the less, Grosz's reputation was not completely overlooked. He appeared on the cover of Life magazine; he had his portrait taken by Robert Penn for Time. "I would be introduced as Grosz's son," says Marty. "The art establishment was so conservative then," he explains. "People would go to museums for a bit of peace and quiet. No one knew about modern art or Dada. All my schoolmates had boats and played tennis at country clubs. They thought my father was shocking."
And dangerous. "Everyone thought we were Jewish immigrants. My father received anti-Semitic hate mail. At the same time we were suspected of being spies. When Germany invaded Czechoslovakia people accused us of being Nazis. It was total paranoia. I mean, this wasn't England. There were hardly Germans leaping out of planes over New York."
Marty became used to acting as a quasi-extension of his father. He even managed to get expelled from high school at the same age as Grosz Senior had. And he showed artistic talent. "I was always drawing, cartoons and things. My father's friends used to pat me on the head and say, 'He's just like his father.' When I was about 17 I even enrolled at the old man's classes. They were wonderful. Packed."
And, by all accounts, fairly laid-back. "My father was a wonderful draughtsman; but in the back of the class people were playing chess," says Marty. "GIs claiming money off the government for college courses. My dad simply said, 'I pay attention to people who really care about drawing. As long as the others pay their fees, I don't care what they do.' And after the classes, he'd give these wonderful talks on art."
But Marty realised it would be impossible to match his father. "It was tough to carry that name. In order to grow, I had to get out of his shadow." When he was a child, he discovered a ukulele in the attic, and strummed the jazz tunes Grosz would whistle around the house. But his interest took hold when he was at boarding school; hitching to hear a big band in Boston caused his expulsion. Grosz was delighted his son was bent on such an unconventional path. "He loved it. I was 15 and my father was having a party, got together with the novelist Ulrich Becher, who had a concertina, and we decided to give an impromptu concert. We played 'Blue Skies' and 'Old Man River'; my parents were so surprised! After that, I was driven. My mother wanted me to go to university, but my father thought everyone should have the chance to do his own thing. He'd come up to my room and have me play along to old records. He'd bring a bottle of wine, two cigars, and his enthusiasm."
On the face of it, growing up with George Grosz was an extraordinary experience. "He was such a performer. He was always doing little songs, putting sheets over his head, acting out stories. And painting. But behind the fun there was a deep embitterment. And his drinking. Drinking, and not working. In the end the periods of not working got longer. Eventually he just stopped."
In the Fifties Eva persuaded George to try living in Berlin once more. "Just for half of each year. Everyone thought he could get his old flair back. But he knew it would never be the same, and it wasn't. He just wondered what the whole point of everything, even art itself, was. Jackson Pollock and those guys were around. My father's day had passed." Including, one suspects, a belief that art combined with a political message could change the world. After what would be his final return to Germany, Grosz collapsed after a drinking bout and died.
He is buried in Berlin, the city with which he'll always be synonymous; yet it is not Grosz, the artist who changed his name to appear more cosmopolitan, but his son Marty who is the true internationalist. "When the time comes, where will I be buried? I have loose views about national boundaries. I don't care. As long as it's not in outer space."
'The Berlin of George Grosz: Drawings, Watercolours and Prints 1912-1930', opens Thurs, RA, London W1 (0171-439 7438) to 8 June; Marty Grosz in Concert at the RA, Sat, 7pm (0171-494 5665). Tom Lubbock will review the exhibition on 25 March.
Rosie Millard is the BBC's Arts CorrespondentReuse content