Well, we talked pretty much as soon as I hit "publish" on this post and this is what came up:
I told him I wanted to support him however he needed and he didn't need to worry about protecting my feelings. (I was worried he was asking me not to go in order to protect me, not because he didn't need support and I don't want my dislike of his dad to prevent me from being there if he needed me.)
He said he wants to have a big, all cards on the table talk with his dad. He doesn't want me or anyone else to come along. He doesn't want his dad to feel ganged up on.
We talked about that and how to convey that information to his dad, so his dad doesn't bring along Homey's stepmom, stepbrother, etc.
I helped him compose a text and we talked about the intention for this meeting.
He's nervous but he's taken the first step to solving the issue on his terms and that's big.
Thanks to everyone who responded last night—y'all give great advice.