Who says cheaters can't change.  But the odds aren't that good.  Will he change if you two marry?  Old habits are hard to change.  If he really want's your trust, then he must not have any secrets like locked cell phones, you get total access to his cell phone and computer, he tells you what he is doing and where he is going, who he is hanging out with.  No, no seccrets.  Of course you should do the same with him.  Both of you must be totally open with each othe for this to work.  If you see any deveiations like no more cell phone access, or his manners has changed, working longer hours.....anything out of the ordinary, it probably means he's cheating again.

In my opinion he needs to show himself not just saying it, he'll change. Don't forgive him immediately, take your time on "hating" him so he came see that you're serious about the relationship . Also make a plan if by any chance he cheats on you again and decides to leave him for good, be strong