Keith Jones's coach was taking a youth rugby team to a game in Burnham-on-Sea on 16 May last year when the child fell on to the M49, near Bristol.
Bristol Crown Court heard the emergency door the boy leaned on was "defective".
Recorder Malcolm Gibney warned Jones, of Bridgend, "he might consider a custodial sentence" on 25 June.
The driver of the coach, Tudor West, 63, also of Bridgend, was found guilty of dangerous driving on Friday.
During the trial he said he had inspected the door on the morning of the incident.
But inspectors found it would open with the lightest of touches and called repairs to the latch on the door "a bodge".
Jurors were also told an MOT test carried out a month earlier had found the wing mirror and panels on the coach were held on by masking tape, and a cupboard was held closed by a pen.
The 13-year-old boy, who cannot be named due to his age, suffered a broken wrist. His father said he "melted inside" when he saw his son lying in the road.
"He appeared to be unconscious - we thought because of the mess he was in he wasn't going to survive," he said.
A coach firm owner has been found guilty of dangerous driving offences after a boy fell from a vehicle on to a motorway.