Hi Kathy,
Here is the most important immediate factor: Have the wedding invitations been sent out?
I’m not minimizing the cheating, but once the invitations have been sent out, 100 people upend their schedule, book flights, buy what’s in the registry, your parents perhaps have booked the venue, etc., etc., etc. In short, one additional incentive to work things out.
If, however, you’re merely “engaged” (without a ring, without a date, “engaged” for several years), then I have a very different answer for you. It’s been nine years. You have a son. If you don’t get married now, nothing will change. If you break up with him for a year, nothing will have changed.
I do, in fact, encourage you to dump him for bad behavior. Yes, he could have dumped you long ago. He didn’t. Tell him this break up (or break, it sounds like) is good for both of you.
Best,
Inky