Confront him; ask him; tell him you suspect him; when he gets mad, tell him you have prior experience and you sense a repeat; tell him he'd be best to be HONEST with you even if he thinks you will not like what he tells you. Tell him cheating is only bad among couples and societies that believe in monogomy (which is NOT the only alternative, even if it's all you know or want, or what you expect). But tell him being a liar is not just bad in particular societies or among monogomist couples its inherently bad and despisable in all societies and among all people and makes him a worthless charcter if he lies to one he supposedly loves.
That might bring out the truth OR convince you he is being honest.
If that does not work and if you can not be happy unless the relatinship is a monogomous one, then you need to break off the relationship and move on ... like the previous poster advised you.
Bottom line: you either get to bottom of this with him and accept what you get OR you change your expectations/relationship with this guy .... as you will make him and yourself miserable going on like you are now .. with SUSPICIONS and UNCERTAINTY on something that is IMPORTANT TO YOU.
rob