That is a very sticky situation. If you're not 100% sure, I wouldn't tell your mom... I couldn't think of anyone else who should confront him but you... I feel very sympathetic that it's a very sad situation, if it is true... I think revealing it sooner than later is better... BUT, I think maybe you should visit a school counselor and get their thoughts on this... I think they are the best to give advice on how to deal and cope with this situation...
best of luck