I will warn, I'm probably going to come off as mean ahead. My intent is to be informative, and I am willing to be so at the expense of tact. For the record, I voted "don't sleep with him."
Firstly, by your letter, you've already decided to sleep with him. You're just upset with what you already know the outcome will be. Actions have consequences, and you know what this action will lead to. Frankly, either get over wanting to sleep with him, or get over being upset at people knowing about it post facto.
Secondly, you don't have any bargaining power in this situation. If you don't have sex with this guy, he can find somebody else. You said so yourself, that he's "hot." I'd like to add, though, you should be asking yourself "why do I think this guy is hot?" I can guess that it's because other women find him attractive, which is a self-feeding, pointless argument. Is he healthy? Does he treat his paramours well afterwards? Does he act TOO nice to people around him? Any of these points are worth thinking about if you can get past your biological urges.
Lastly, for now, I'd like to point out that you're not going to change him. You're NOT going to be special to him. You're not filled with magic or the protagonist of some young-adult romantic novel. In the market place of sex, you should be able to find a guy who better suits your desires, either one who desires a relationship, or one who cares about discretion.
Don't believe a guy who says he'll leave somebody for you, because either he'll do it to you, or he's lying; either way, he's already a cheater. Pay attention to past behaviors, how they treat others (esp. waiters and service people), and how upfront they are about things. This guy, at least, isn't hiding who he is or what will happen, but you obviously don't want that. Shop around for something better, or get over your hangups.