You should tell him, all calm and careless, "I dont care. You have nothing on me, It's only my ***. Have fun with it on the internet." and walk away.
Dont panic, no big deal, no ones going to see it and know its you.
If he tells someone its you and they ask, just deny it, because its no proof.
And get rid of the underwear you were wearing.
Never date that much of an older guy, and never talk to him again. If it gets serious, tell an adult.
And if you tell one, dont say, "he has a pic of me in my underwear." say "he said he'll get a fake pic of me in my underwear and put it on the internet." Just deny it.
Wtvr you do, dont do it with him. It would be rape.
ANd dont cry, dont panic. No need.