Toothache - can be from a number of reasons.

But if he has seen a dentist they should of been able to rectified the problem.
if not please see one immediately.
as it has been 3 years this is a chronic problem which should be dealt with
another solution is reflexology if you apply pressure to all of his toes
and I dont mean standing on his foot,
if you apply pressure to his toes - this will send a pain relief receptor to his brain and eliminate
most of the pain. but I still stand by my original idea and have it checked out by a dentist.