I'll tell you a guy-secret Jodi: We don't have friends who are girls. We have friends who are guys, and we have girls we are currently just friendly with but we haven't taken it to the next level yet. Now, sometimes we don't have any plans to move past friendship with a particular girl, but that's always subject to change. Most of the time, we just haven't seen a good opportunity.
This doesn't mean you can just have sex with him and then go back to being friends. Guys are actually more complex than that. Once you have sex with him, he will feel possessive / protective of you, and he'll want to keep having sex with you. Anything else will make your friendship feel awkward and eventually ruin it.
My advice to you is:
(a) Don't do this unless you fully intend to keep doing it until one of you is in a monogamous relationship.
(b) If the sex isn't that great, you won't want to do (a), which means the friendship will most likely be over.
(c) Don't have sex with him first and then later say that it was just for fun. Most guys can handle that if they go in knowing it's just for fun, but unless it's really his idea he'll expect that it's leading to a more-than-friends relationship, and finding out otherwise will be a blow to his ego and will make the friendship awkward until it dies.
(d) Don't try to manipulate him into thinking it's his idea. Any guy worth doing this to isn't stupid enough to fall for it. You'll just confuse him with mixed signals, and he'll probably back away from you to try and protect the friendship.
(e) If you really want to do it, be straightforward. Tell him "I'm not ready to be in a committed relationship, but the other night was really hot and I want more. Do you think our friendship would be ok if we took it further?" Give him time to make up his mind; it could be 10 seconds, or it could be a couple of days.