Is this considered cheating? What should I do?

Don't dump him do some investigating yourself invite him over when he leaves the room look at his text messages, when your at his house look on his computer to see if he didn't log out of facebook. If you hang out around this girl call him over if he looks nervous hit him with questions most likely he will tell the truth but first get your friends to surround him when he comes over and bang you have your answer.