I know what you're feeling, having been there many times myself. With him, the details would come out in spurts, usually when he was dissociating, with little or no time to brace myself. I chose to look at it from the perspective of...if it was difficult for me to HEAR it, how difficult must it be for him to SAY it? He wasn't committed to therapy, he wasn't telling anyone else...and I am a firm believer in "better out than in". What used to break my heart the most, is that after he would disclose something, he'd follow it up with "Do you think bad of me?" As if that could EVER happen...
Our relationship ended badly, after 30 years. But I stand committed to being "Pro-disclosure". It prevents so many "What the F@#$?" moments in the relationship. These are things someone did to him, they aren't ABOUT him. I have alot of anger at his uncle, more than he seemingly has. Whenever he used to talk about how much this man "loved" him... (Talk about wanting to wretch!)
This is something that you have to work out with him. You've indicated that he's seeing a therapist, so he has an outlet for discussion. You have EVERY right to set boundaries for yourself.
I wish all the best for both of you.