Personally, I don't think your being naive is a abd thing - the fact you can come out and ask thse questions is definitely a step in the right direction!
First off, I"m not so sure how to answer your first question. If it's going to be a regular thing, I could see your reputation getting a little tarnished if you aren't going out with him. Is it just a sexual attraction bewteen the two of you? Could there be something more? If so, why not consider a relationship with the guy?
Secondly, using a condom never hurts. It's always better to be safe than sorry (even if he moans like hell about having to use one - make him use one anyway). If you plan on having sex with him regularly, why not ask your GP to put you on the contraceptive pill? Hwoever, this doesn't protect against STI's, so using a condom ontop of the Pill, seeing as it is casual sex is always a good thing.
Finalement, it's not wrong that you want to sleep with him. It's natural for a start. I can understand about your quarms about not being his girlfriend or fiancee or whatever, but wanting to sleep with someone is not wrong. Besides, you're more than old enough, and you're certainly old enough to know what you want. As long as you both state that it's just for sex and don't get attached then emotions don't get involved. If you both begin to feel for each other in a deeper sense - bonus! Not being afraid to go out and get what you want is definitely not a bad thing. The people who say that it is are just jealous.
BUT remember to be careful!!