How do I handle this conversation?

these are Facebook messages with a couple hours between each one, so this is an ongoing conversation right now.

I asked him how his trip to Puerto Rico was & he said it was great & talked about one night out in a boat in a bay. it was bioluminescent or something like that. anyways, we talked about it for a bit & I asked him if he had any more interesting stories.

He says: oh gosh yes. like...i could tell one every second.

I say: I'm all ears :)

he says: hmmm I'll try to do one every time I get on. lets start with me having sex on the beach.

I say: what? haha

he says: you heard me :)

I'm at a loss of words. We're the same age (18) but I view him like a little brother. and he's athletic & smart & attractive & all...but he's not the kind of guy to do something like that...on a school trip (or at least that's what I thought). I can't tell if he's joking. and if he really is telling the truth, why the heck is he telling me this...

if he's joking, I'd like to (carefully) joke back as well. if he really did do what he said...what do I say back? my (female) friends & I don't really talk about sex, so this is a new one for addition, my conversations with him have never strayed past school & family vacations & general chit chat...

I'd appreciate any insight/advice on his end of the conversation & on how I should respond...
How do I handle this conversation?
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