Be careful. Don`t want to scare you, but if your mother confronts him, he could be the type to turn everthing on you. I would try to have someone other than your mother to stand at a distance and observe him doing something with you for you to have support and defense. I know that`s hard. Maybe a small tape recorder, make yourself available for him to do it again and record him. Let your mom listen and ask her what she thinks. You won`t hurt your mom as much now as she could be later if things continue.