Hugh McDaniel, 55, was found guilty on two out of five charges of abusing the boy in the 1970s.
A three day trial at Letterkenny Circuit Court heard how McDaniel befriended the boy in the summer of 1979.
McDaniel, of Abbottswood, Carndonagh had pleaded not guilty to all the charges.
Judge John O’Hagan gave an order not to identify the victim.
The victim, who is now married, gave evidence during the course of the trial.
He said he had never told anybody about the assaults but broke the news to his wife before they married.
He later told his sister.
He alleged that McDaniel indecently assaulted him on four occasions at two quarries in Inishowen and also at a house.
After deliberating for more than two hours a jury found the defendant guilty of on two counts of indecent assault, failed to reach verdicts on another two and found the defendant not guilty on a fifth charge.
McDaniel will be sentenced at the next sitting of the Circuit Court in October.