You're in a really tough position. No matter what, you're going to appear like the jealous ex. I know it's foolish-but that's how people will view it-because that's the easy way-looking at it realistically, they'd have to DO something about it, and who wants that?
I can see exactly why you're so concerned. I'd be scared something would happen too. Just because he's on probation doesn't mean he won't come for a visit.
As "mean" as you might look, I'd put my foot down & not allow my son over there while she's there. Especially considering she doesn't want his father paying attention to him. Tell your ex, "You can focus 100% of your attention on your gf when our son isn't there. Then, whenever you want to see him, you can spend quality time with him and she won't need to get jealous. Instead, she can use that time to spend quality time with her kids."
Doesn't every good parent want their future possible partner to be good parents to their own as well? Blame it all on the gf's ex if you have to, then there'll be less that can be said about your "jealousy"

OR-do you have any acquaintances you could hook your ex up with? lol...you never know! If he needs to have a woman in his life, it may as well be someone you approve of!
