if he has actually cheated already,i think you should talk to your him first.i know this is gonna be a really unpleasant conversation but it needs to be done.
1)you must warn him not to do it again otherwise you are going to tell your mom (if he doesn't show some remorse and continues with that)
2)if he continues with it,go tell your mom.the only way she can handle this is to "have a little chat" with him,find out what's wrong in their relationship etc.she may decide to forgive him after all.
3) [i hope it will never come to this] if he still continues,even after their "little chat" thing,the only solution is the divorce
i'm terribly sorry about your situation!!please,keep us updated...