Spoiler for his role and no, unfortunately he does not take his clothes off:
He plays the Ivy League son of a rich businessman who, after getting an introduction to one of the main characters Regan, rapes her when she's drunk (this happens off-camera) and gets her pregnant. This is all a flashback and is a catalyst to some of the character-related events in the show. I think, and this is just from my memory of the book, another main character Charlie is their son who was put up for adoption. I don't think it was explicitly said in the book (only that Regan had a baby and that Charlie was adopted) so that might have actually changed, because it's a 15-year ago flashback and I don't know the age of Charlie off the top of my head other than he's a teen. Either way, John only appears for three scenes (has a speaking role in all three of them this time) and it's his biggest TV role to date, in my opinion. Too bad he's playing a rapist asshole but hey, someone has to.
I literally bought Apple TV+ for this so everyone make sure to thank me lol. There's only two more episodes left but I'm not sure he's in them, as his IMDB page only lists this one but I'll check next Friday and the Friday after anyway.