My way to have him never annoy me. Do Thaniel's quest WITHOUT having him in the party, done, saved the boy good Act 2 ending. And yes, without Halsin, no saving the kid. Then Ignore his existence, never even talk to him, imagine him just as what he really is, a forced NPC you must have in camp to have a certain quest done. I did it, and boom, all my problems were solved.
Still annoys me having to avoid any npc, but honestly, I didn't miss a thing not interacting with the unpleasant horny weirdo dude.

Also, while in the poly thing, isn't that unfair that they gave (forced) a dude option, but didn't have a female option for those who want poly but not a man?