I wonder what kind of mates the guy has if they told you that he is cheating. Normally guys do not defect on each other, even if that is true. Could it be that his mates don't want you hanging around him? That's something you have to figure out on your own I am afraid. As far as him avoiding you is concerned I guess it would not matter if he is cheating on you, would it? He probably does not want to be with you anymore but does not have the gall to be honest to you. Besides, the more you chase after him the farther he would go. I suggest you best move on and forget about him. There's no point in hurting yourself over someone who does not really care about you.
