My 351C Cougar, after lending it to my son who thrashed it for 9 years came home running like s__t, and popping out of the pipes. Does that sound like fried exhaust valves like it does to me? He mentioned getting it hot a "few" times, throwing a belt and never changing the oil FOR 9 YEARS!!!
I think it's time you teach your son the finer point of engine rebuild. Up close and personal. It could be carbon build up. The oil has gotta be as thick and dark as tar. How many miles did he put on it?
After what you described, you at least have to pull the heads and drop the pan to have a look.