I'd go straight for his senses. Through whatever method you prefer, get his penis out, as if you are going to masturbate him or give him oral sex. Stroke him with one hand and with the other hand rub your fingers around the head to "catch the smell". Bring the smelly hand to his nose or mouth, then tell him you would have been able to finish him if it was clean.
Seriously, I'm a guy, and bisexual. I hate the smell (and taste) of uncircumsized guy who don't clean down there. I personally got circumsized at 28 because even though I had no disease or problems, I always hated the basic scent I had and couldn't fathom my wife putting my penis in her mouth with that smell.