Get proof before you confront him... Nobody likes to be accused if they are innocent. Get the proof and hang him by that cheating organ. ;)
Well how do you know that he is cheating. The fact that you are so sure about it tells me that he is not doing things like he use to do it. He comes home late. Go out with the guys a lot more. He change his appearance. Even sexually he would change. He would treat you different. If that is the case then that is already enough proof to confront him about your suspicions. Don't fight with him. Talk to him and look at his reaction. The way he will react when you talk to him will give you the proof
If you know for a FACT that he is cheating then get proof! So when you confront him you have something to put in front of his face and tell him straight up like dude what the hell are you thinking!!! But you always need proof first!!! If he has a myspace go on the I'm sure you can get proof from that or emails the history on the computer and maybe even on facebook! Those are all places you can get your proof from that's what my mom did!
Be to the point, stern, and stand your ground you know he is cheating and do not bring up the fact you have no proof. By the way are you going on a hunch he is or do you know he is for sure?