I would say something along the lines of "that's not appropriate" or "that is rude!", and then leave (or turn away from him, perhaps keeping him in your peripheral vision for safety).
Remember, if he touches you after making such a comment, that may be cause for a complaint to the police.
Fwiw, I've had two incidents like that...the first was a neighbor who asked point-blank, "Are those real?" and then poking a breast...I excused myself and left. My other neighbor friend (also a male) then proceeded to...umm...verbally correct the other guys behavior, with a mild emphasis on physical punctuation. (Bad guy ended up with a broken thumb)
Second incident was when I was talking to someone about transitioning, social roles, expectations, etc when he blurted out, "I bet you like playing with your boobs!" I coldly said, "That's none of your business!" and ended the conversation.