I'm sorry but it doesn't sound like you're ready to be in a relationship. This soon in, this is a huge level of stress - it shouldn't be this hard.
I'm also concerned you mentioned your son thinking he is a great guy and you wanting him around so much. It sounds like you've involved him very quickly in family life and I don't think that's healthy.
You really should focus on developing a strong foundation with a partner before introducing the kids and it's already gone a bit tits up.
As his behaviour has suddenly changed I've gone at him tooth and nail your cheating I actually reduced this poor man to tears trying to convince me he isn't.
This is incredibly abusive. If a woman posted on here saying her boyfriend had "gone at her tooth and nail" accusing her of cheating until she cried what would you think? Because that's exactly what you've done to him.
I'm sorry OP I don't mean to have a go but this is a toxic and unhealthy situation and for your sake, his sake and most importantly your sons sake I think you need to draw a line under this relationship and do some work to build your confidence and coping mechanisms so this doesn't happen again.
If you are sure enough he's cheating to shout at him until he cries, then you must have believed it strongly, in which case someone with health boundaries would end the relationship if they thought their partner had definitely cheated.
This is all too much stress, too soon and too far gone to fix.