Hi Shaz, I am normally a lurker on here but had to post, especially as I got so much wonderful support from MN when all of this happened to me a few months ago.
There are loads of similarities between what you're going through and what happened to me. I found the advice here to be mostly sound but you have to also follow your own instincts. I was never comfortable with snooping. After I told my H to either ditch the OW or leave he chose to leave. I stalked him via his e-billing for his mobile and attempted to hack in to his email but the more I found out the more it hurt. He made his decision (he is a romantic fool, always has been) and was obviously torturing himself with 'What if?' - the two most dangerous words in the English language, if you ask me! - so he had to go and find out if this relationship was all he thought it was.
Several months on I am still hurting like hell but it DOES get easier. I don't ask him what is going on with her, I haven't a clue whether they are still together - it doesn't really matter. He didn't want to save our 14 year relationship or stay under the same roof as me and our daughter, what else do I need to know? When I look back I realise that there were loads of flaws in our relationship - you are probably doing the same. That doesn't mean it was a bad relationship, so don't let his actions taint your memory (this is hard, I know!) It seems to me that some people (not just women - I know men who are like this too) are much better at accepting that you can't have a lifetime of excitement/passion/romance - the teenage feeling goes away and is replaced with a feeling of companionship and comfort. Some people see this as an excuse to start affairs and then try to persuade their devastated partner that it was beyond their control, that they can't help it, 'I'm not in love with you any more'. All bollocks if you ask me!
But what I am trying to say is that you can get through this - it's fucking hard, but you can do it. And your lovely little boy will help you - kids are a wonderful distraction. You are worth more than this and you are doing so well to stay dignified and not go round and kick the shit out of either of them. You won't regret keeping your dignity! (IMHO you will regret using your son as a weapon so try not to restrict access too much for the sake of it).
Anyway, keep your chin up and keep posting here. Sending you lots of love & hugs.