Dunno - you’ve got to play it in your own style, really, but maybe I’d ask him if a picture of someone else’s breasts would do.
Send him a photo of your dinner. Before cooking it.
I might make it my goal to get a picture of his cock without sending anything myself.
don’t ask, just send him a playboy picture of like, Carmen Electra or Cindy Margolis or something. See if he notices.
I’d play along- answer all his questions, play nice, then see if he’ll send you a pic of his wiener, and in return-- just like you offered-- send a picture of your bare chest. 
Actually, you could use the old fishmonger/kipper line: tell him “I don’t have a pair of breasts - will two odd ones do?”
I might be flogging a dead horse here, but on the offchance that the scammer hasn’t tumbled that Mange and Dastardly are also taking the piss, Dastardly has written to ‘Patrick’: