I can't speak for your boyfriend, but I can say that I have experienced paranoia and delusional thinking before do to episodes. When I get really manic I start to get paranoid that people are watching me and eventually I start believe the paranoid thoughts are true, no matter what people do or say contrary to those believes. It could just be that his paranoid thinking and/or delusional thinking is that you're cheating. No matter what you do or say to challenge his believes he's still going to believe it until he gets the proper help. I know that whenever I start to have these the wife has to convince me to call the doctor or up my meds. I wish there was something else I could suggest.

On a side note, he probably shouldn't be smoking weed because it is known to cause episodes or make them worse.