You can't do anything if he cheats that's the way things are... All you can do to ease your mind would be by having a good communication with him as often as possible even though he's getting bored with it. Try not to over obsess, do something that can take off your mind about this kind of things but I just want to remind you about the saying "Fool me once shame on you fool me twice shame on me but fool me three times it's stupidity",(the last part is just a modified version)...