Although there is no proof he is cheating just because his friends have other girls hanging around it sure would be enticing for him and he shows it by acting different. The best thing you can do the next time you see him is express how you feel and come right out and ask him. If he tries to play dumb, don't let him get away with it. Simply ask him why he prefers to spend more time with his friends than he does you. If he can't come up with an answer you believe and you don't trust him then it's time to move on.