Nathan Luke Housecroft, 19, of Trinity Road, Bridlington and two 15-year-old boys, who cannot be named for legal reasons, appeared at Beverley Magistrates Court on April 10.
They were charged with the assault of a 15 year-old boy on Vernon Road, Bridlington on August, 21, 2018.
Detective Sergeant Carl Sweeting of Bridlington CID said: “This was a sickening and horrific assault where two teenagers and an adult took a vulnerable 15-year-old boy with learning difficulties down a lane and punched and kicked him for no reason other than thinking it was funny.
“The victim was punched and kicked repeatedly until he fell to the floor where he was then stamped on.
“Fearing it would make matters worse the victim did not defend himself and didn’t throw one punch in retaliation."
All three accused pleaded guilty to the assault.
Housecroft received a 26 week prison sentence suspended for two years, a 30 day probation service rehabilitation course and ordered to pay £250 compensation to the victim.
The two 15-year-old boys were both given a 12-month intensive referral order along with 30 hours reparation and £250 compensation to the victim.
The judge warned one of the youths that if he breaches the order and returns to court he will be looking at a detention and training order of 6 months.
DS Sweeting added: “This was a particularly nasty incident and one which the investigating Officer, PC Sebastian Petch was determined to bring those responsible to justice for the victim and his family.”