The first thing I want to say is your husband is a very, very lucky man to have such a caring and attentive wife. He's going to need your patience and support while he processes and remembers his sexual abuse. It's very good he's willing to enter therapy. Some men prefer to suffer in silence.

As far as telling his mom goes, it's not something I would do. If he tells her she would probably just deny it, call him crazy, and likely blame it all on you. That's a scene that wouldn't be conducive to proper healing from trauma. But no matter what your H decides to do, continue to support him. It can be a rough ride sometimes. You could also keep posting here for your own support and maybe, when he's ready, your H can get things off his chest here as well.

Hugs to you both.