The boy is believed to be with his mother, who vanished last month after admitting to having an affair.
Glenn Arthur, a successful 43-year-old businessman who earns pounds 300,000 a year, says that his six cars and six houses are worthless without his son, Blake, whom he last saw a month ago.
His 25-year-old girlfriend, Nathalie Berquez, is believed to have run away last month with the boy and her new lover, Craig Cook.
The couple had been together for five years but split up in June after Mr Arthur discovered that Nathalie was having an affair.
She left the home they shared in South Norwood, London, but they had worked out an amicable arrangement whereby he saw his son three times each week.
Mr Arthur is reported as saying even his partner admitted he was closer to the boy than she was. Mr Arthur said: "I never went for more than two days without seeing him. Now it's been 29 days, and I'm going crazy."
The search began three weeks ago, when he managed to get a High Court order demanding Blake's return pending a custody hearing, but got no response.
Since then he has used the poster company he runs to produce huge posters, appealing for information about Blake's whereabouts, which he has put around Brighton where the three were last seen.
Mr Arthur said he would gladly offer far more money to see his son again, but is worried in case an even bigger reward prompted rescuers to take some rash action.
He said: "Initially I offered pounds 25,000, then pounds 50,000, but I think this is as far as I can safely go. I own six cars, and I make very good money.
"But I'd give it all away to get Blake back. He's my life."Reuse content