Hehe, to be young again. :P
I can tell you from experience, that this won't work. I mean, you can say lets fuck, and he'll say sure.. But then (and especially because you haven't been apart very long) you're back together. Period. Even if you do a lot of upfront crap about how this is just physical, having sex with people kicks in hormones in your brain that make it more than physical. Especially if it's an ex.
I'd say that you should find a new guy if you want to stay broken up, lord knows, there's no shortage of guys out there that wouldn't mind some casual sex.