Christopher Hawkins, 47, of Slaithwaite, near Huddersfield, West Yorkshire, told his two daughters he loved them as he was led away from the dock at Leeds Crown Court.
He was sentenced to a minimum of 21 years in prison for stabbing Ryan to death.
He was also given a 12-year sentence to run concurrently for attempting to murder his 14-year-old daughter Donna in the same attack at his home on September 23 last year.
Hawkins stabbed his son nine times during the attack and he was killed by wounds to his chest, two of which had penetrated his heart.
Sentencing Hawkins, Mr Justice Wilkie said: "A more horrific crime it is hard to imagine than savagely and repeatedly to stab your young son who, in his agony, had to try to fend off blows from one of the people he was most entitled to expect love and affection.
"Although to this court, and after the event, you profess great love for Ryan and mourned his passing, in my judgment, having seen you give evidence and had the opportunity to assess your genuineness, these were crocodile tears."
Sheet metal worker Hawkins and his wife, Valerie Gee, had separated in May last year after she told him she no longer loved him.
He later discovered that she had begun an affair with taxi driver Lee Tinker, whom she met at her uncle's cab firm where she worked, while they were still living together and was described as being "extremely unhappy" about the relationship.
The day before the attack would have been his and Miss Gee's 17th wedding anniversary.
The judge said: "I'm satisfied that you used him (Ryan) cold-heartedly as a vehicle for avenging yourself on your wife."
He praised Donna for the calm way she gave evidence during the eight-day trial and said it was in "stark contrast" to the performance of Hawkins, who he accused of not having the "moral courage" to face up to what he did, instead blaming "something evil" inside him.
"Your daughter, by the way she gave her evidence, showed herself to be a credit to herself, her mother and her sister. This is the more so when one is aware of the pain and difficulties she has been subject to," Mr Justice Wilkie said.
Donna suffered multiple injuries to her face, chest, abdomen, arm and thigh.
Two of the wounds had penetrated her abdomen with sufficient force to also penetrate her liver and one deflated her lung.
Hawkins had also "cut a chunk" out of his daughter's right arm.
The judge said: "I'm satisfied, but for the speed and skill of the doctors at the hospital, you would have faced two murder charges."
He added that he did not accept that the attack was spontaneous as Hawkins had written a note stating that he and Ryan would die on September 16 - Ryan's fourth birthday.
And, after attacking Donna, he told her he was going to kill Ryan.
As he was led away, Hawkins, wearing glasses, a dark suit and tie and a white, striped shirt, turned to the packed public gallery, where daughters Donna and Natalie, 16, were sitting, and said: "I love you, Donna, I love you, Natalie."
In a victim statement read out to the court, Miss Gee told how her world had fallen apart when Ryan died.
"I was angry, I was in a daze," she said in her statement.
She described her son as a "loving child" and told how he used to hold her hand and show her constant physical affection.
"We played, laughed and life was more enjoyable and bearable for having him around," she said.
She continued: "The last words I exchanged with Ryan were 'I love you'. These are the words which will remain with me forever and sum up my relationship with my son."
Miss Gee said she has a corner of her lounge dedicated to Ryan, pictures of him all around the house and all of his clothes, washed and ironed and "ready to wear as if he was coming back".
She described having to tell Donna that her little brother had died as she was seriously injured in her hospital bed.
"Donna had been asking where Ryan was. I remember telling her that he was with us," she said.
She added that Donna was physically and emotionally scarred for life by the attack and would never be able to get over what had happened.
Speaking about Hawkins in a statement read out outside the court after the sentencing, Miss Gee said: "He deserves everything he has got today for murdering my son, Ryan, and attempting to murder my daughter, Donna. I don't feel sorry for him at all.
"No amount of time in prison will ever equal the pain and suffering we've got to go through every day without Ryan in our lives."
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