I would ask your son what HE wants. If he prefers you to tell H, I would do that - heck, we moms need to buffer our kids from the crazy H's. If you think your H is going to go off on son, well, he doesn't really need that right now, does he?
If son wants to tell H himself, then offer your support - if he wants you there, fine, if not, fine. Just remind him that H's response is likely to have more to do with how messed up H is, and little to do with son.
