The first thing you should do is ask him why he kissed another girl. If he really did it, there must be a good explanation to that. You should then tell him how that makes you feel. Ask him if he needs a break so that he can think things over. You should also decide if you want to continue with him or not.
Your still calling him your boyfriend?! No! Break up with him! But act as if it doesn't bother you. Just go up to him and be all like "Wow, you went on longer than I thought! Sorry but break your heart but its over.." And then walk away! If i ever saw a girl say that to a cheater i would be all like "Wo Hoo! Go girl!" :P Hope I helped :)