I would absolutely tell him. It will be hard and im sure he will have to work through it, but I cant image that he is gonna go through is whole life without ever finding out. If you go through something like its best to hear it from someone he knows he can trust, especially if that side of the family is gonna be back in his life, he deserves the right to decide for himself how close he is comfortable being with them.
There's never gonna be a "right time". You just have to it. And if he's angry or hurt, just let him feel it for a moment. There has been a lot of this stuff in my family (Not by family members) but it has happened to many women in my family, and they all workes through it in their own time with people they could trust and fall back on when needed. And I say this 100% honestly, that they are better and stronger for it.
And to, if he was that young and can't remember it, it might be a lot easier to deal with and move on. Be he needs to know in order to put it behind him.