I would have to say yes they can change because I used to be the biggest cheater and so did my gf this is both our first monogamous relationship and it is the best sex either of us has ever had because we give all of ourselves to each other only and it means so much more
I was with my husband, he was a serial cheater, he won't change, part of who he is, not with him now. This guy, how many times did he cheat? One night stands or flings here and there? Maybe he was unhappy in his relationship and so will be happy with you now. Or maybe he is addicted to the thrill of it, in which case he probably will repeat past behaviour.
I have always been the believer one who cheats will cheat again but I think that it depends on the circumstances....and while I would never condone any cheating....I do think there may be some who truly regret what they did and consciously make the decision to NOT hurt another person that way again.
I was one to truly regret it and paid the inital price 10 years ago. But was able to totally change and realize we both had some underlining problems and through counseling we managed to get back on track. Coming up to our 20th ann. now and never have since.
Anyone can change but few will.