Andrew Wragg never denied having killed his son Jacob after smothering him with a pillow in July 2004.
Andrew Wragg said it was 'insulting' to suggest killing Jacob was 'selfish'
But the man described in court as a "brilliant" father denied murdering the boy, saying it was a mercy killing to end the youngster's suffering.
The 10-year-old had Hunter Syndrome and Wragg told the court he thought killing him was the best thing to do - and claimed Jacob believed that too.
A jury agreed with Wragg's version of events and cleared him of murder.
The former SAS soldier, 38, phoned the police to tell them he had killed his son and admitted manslaughter on the grounds of diminished responsibility, for which he was eventually given a two-year suspended sentence.
'Brilliant with kids'
He said in court: "I have put my hands up to taking my son's life."
Jacob had Hunter Syndrome, a degenerative illness for which there is no cure and which meant he was expected to die by the time he was in his 20s. He was also deaf and almost dumb.
Wragg, from Worthing in West Sussex, told the jury that before killing his son he looked into his eyes and believed the boy could no longer recognise him.
He told how he asked him: "Have you had enough son?" before suffocating him and sitting cuddling his lifeless body.
He said: "I picked up a pillow and placed it over his face and I lay on top of him and he passed away.
"I was just cuddling him and I was talking to him, trying to explain.
"It is insulting to me and to Jacob to suggest that what I did was selfish."
Shelley Ansell, a former girlfriend of Wragg, told the court how he loved his children.
"He loved spending time with them. Jacob doted on his father.
Christopher Wragg (right) said his brother and Jacob were very close
"Jacob was very affectionate but more to Andy than anyone else. He really was a loving father."
The defendant's brother Christopher and sister Christina Chandler both told the court what a good father he was, to Jacob and his younger son George.
His brother said Wragg had an "amazing" relationship with Jacob, saying: "They were very close and it was mutual."
Mrs Chandler said: "I know Andy could be difficult to live with at times but despite this he was brilliant with the kids.
"He was a wonderful father and you can never take that away from him."
Wragg's claims that the boy's death was a mercy killing were backed up by his mother-in-law, Jacob's grandmother, Gwendoline Richards.
Mrs Richards said she had not wanted to see Jacob die but felt his death was "a release from being ill".
The court heard that on learning of his death, she told police: " To hear this was the best news I could have. He was not Jacob any more, he was just a shell."
The prosecution claimed Wragg's killing of his son was a selfish act by a father who could no longer cope.
But the jury at Lewes Crown Court agreed with the "brilliant father" and cleared him of murder.