It's up to your son whether to prosecute (assuming he is over 18?), but because it's been so long it would be hard to prove it unless the pedophile admitted it, which is doubtful.

Other then that, I agree with everything Rhap said. Take care of yourself, support your son, but try not to push him too much with this issue. Maybe ask him if he's willing to talk to somebody but that you'd understand if he needs time and you aren't going to make him.