You're a bit young, so I'm guessing that your 'friends with benefits' situation doesn't involve THAT kind of benefits. I'm not here to judge so I don't care, but at least be careful with it if you're getting serious. Life is just as real at 13 as it is at any other age, and so are the risks if you know what I mean.
Anyway, here's the deal. You don't need 'good' dirty questions to turn a guy on, you just need dirty questions. They don't even have to be really dirty in fact, just suggestive. Guys all have completely dirty minds (you don't ever wanna know what goes on in there), so all it takes is the slightest suggestion and we're primed and ready. Seriously, you shouldn't have to work to get a guy's motor running for you. All you really need to do is show up, and if that doesn't work just undo one button. If that doesn't work, he's either dead or gay.
When I say be careful, I also mean don't give him the idea that you're willing to do something that you're not 100% willing to do. A lot of possible things can happen if you do that (it's called teasing), and none of them are good. You don't want that kind of hassle, especially not when all you wanted to do was have a little harmless fun.
But as long as you keep it reasonable and don't lead him too far, you'll have a good time. For the final question, ask him exactly what he would do if he could have half an hour with your body and you would let him do anything. Prepare for a really graphic answer if you ask that, but it's a good one.