how about you just trust him. and if you really wish to find out. sit down and talk to him. it's better to fix your own problems than have everyone else try to. youre the only person who knows how you truly feel about him. Honesty and trust are the keys to happiness. just talk to him.
Okay I am not being hateful or anything, but you could have given some details of how he is acting toward you like if his attitude and affection to you.... Anyways I would have to say if he has turned hateful toward you or assumes you are cheating on him when really it could be him doing so. Sorry to hear if he is and wish you LUCK on finding out too !