Jordan Potts, 22, of Rhyl, Denbighshire, accused innocent friend Jamie Jones of seeing his girlfriend.
In a rage, Potts forced his way into his friend's home and inflicted "devastating violence" as Mr Jones slept with his two-year-old.
Potts admitted grievous bodily harm and burglary and was jailed for three years at Mold Crown Court.
Mr Jones suffered devastating consequences and was still affected by the attack, which happened in April.
Judge Philip Richards said: "You were prepared to use devastating violence, not on a stranger, but on a young man who had offered you friendship.
"It beggars belief that anyone could behave like that, towards his own friend."
Father-of-one Potts had been seeing his girlfriend for two years, but had heard rumours she was seeing Mr Jones.
It was not true, but on 12 April Potts forced his way into Mr Jones's house, barged into his bedroom where he was asleep with his son and attacked him.
In a victim impact statement, Mr Jones said he has slurred speech and dizziness as a result of the attack, and has been warned by doctors another serious impact to the head could kill him.
A jealous boyfriend beat a man so badly in front of his toddler son another serious injury could kill him.