Welcome to the boards, AS83.
Your boyfriend disclosed to you five and a half years ago, and you've stuck by him this far, so he probably trusts you pretty well.
My boyfriend took close to a year after disclosing his abuse to get therapy, but when I look back on that year and the time directly prior to his disclosure, I can see that he was already taking steps to change his behaviors and be healthier.
After living with the "old" boyfriend for so long, I wasn't looking for some of these positive changes and I was slow to notice some of them, which probably felt discouraging to him. One great thing you can do for your boyfriend is be on the lookout for positive changes and let him know when you see them.
For my boyfriend and I, we don't talk much about what goes on in his therapy. Sometimes he'll want to share something with me, but usually he's not looking for advice, just a friend to listen. More often, when he comes back from the appointment he wants a lot of space, sometimes for the rest of the day, so I put the kids to bed, answer the phone, etc. and I know he appreciates that.
Good luck to both of you