Don't let him, Ed. For your own sanity, just veto the character.
Yeah, this is something a lot of novice DMs have trouble learning: if a proposed character clashes violently with the type of game you're trying to run (e.g. a literal cartoon character a la Who Framed Roger Rabbit? in a relatively serious, straight-faced campaign) it's perfectly fine to, perhaps not hard-veto it, but tell that player "Yeah, maybe save that one for a later campaign." If they throw a fit over it, it's their problem, and if they insist on making it everyone's problem, kick 'em out until they grow up. (Context is important, of course: if everyone -- and I mean everyone -- shows up with a cartoon character, then just accept that that's what your group's in the mood for. If their ideas are all over the place, though, it's perfectly reasonable to tell them to get all their metaphorical shit into one sock before proceeding.)
This is also a good reason to make character creation a group activity: it gives the DM a chance to catch character proposals that'd be so strongly opposed as to make it implausible for the two to be in the same party, even temporarily.