Do you want to break up with your boyfriend? If that's the case, then tell him ASAP. Get it all out in the open and go your separate ways.
If you don't want to break up with him, then that's a whole other story. Why do you want to tell him? Is it because you feel guilty? Confession may be good for the soul, but it's not always good for the relationship. If it was just a little hook up and now you feel bad and you don't think it will happen again, then keep it to yourself. The guilt will be punishment enough.
(One side note to this: do any of your mutual friends know about the hookup? If there is a STRONG possibility that someone else will tell him, you have two choices. Either break the news to him gently before someone else does it or wait to see what happens...then, if someone else tells him about it, you can say that they don't know what they're talking about!)
