Tudor West was taking a youth rugby team to a game in Burnham-on-Sea on 16 May last year when the child fell from the coach on the M49, near Bristol.
The 13-year-old, who cannot be named, suffered a broken wrist.
Mr West, 63, of Bridgend, was convicted by a jury at Bristol Crown Court of dangerous driving. It has yet to reach a verdict on company owner Keith Jones.
Mr Jones, also from Bridged, denies permitting the driving of a vehicle in a dangerous condition, and aiding and abetting dangerous driving.
During the trial, jurors heard claims that the emergency door on the coach which the boy fell out of after leaning on it was "defective".
The prosecution claimed the defendants "ignored an obvious risk that could have had tragic consequences".
The court was told the wing mirror and panels on the coach were held on by masking tape and a cupboard was held closed by a pen.
Mr West had been a coach driver for more than 10 years and had no previous convictions.
A coach driver has been found guilty of dangerous driving after a boy fell from his vehicle onto a motorway.