(This from an amateur testing his chops) I think your question comes down to, “what’s a clear argument I can make against hidden variables?” And if your protagonist is familiar with Bell’s theorem, then he must have an argument for that. I can’t imagine what that is since the matter seams to be settled. So maybe it has something to do with the locality of those hidden variables. So I’ll mention the Bohmian interpretation, which has advantages like many worlds in that it can simplify things. It’s considered to be a hidden variable interpretation but those variables are also non-local. The Bohmian interpretation is considered to be legitimate although it’s still just an interpretation. There are other interpretations containing non-local hidden variables. (See
Hidden variable theory)