I'm sorry that you're here, brokenapart, but I'm glad that you found us.
I can relate to your situation as my STBXH moved in with his posOW. We don't have kids, though, so I can only imagine the hurt and pain you're feeling right now.
I agree that you can't control what your STBXH is doing with your son. But you can control what you do with your son. I like Elegirl's suggestion of taking your son to a professional to get him through this period, if necessary.
And, yes, you can handle this! Keep posting here. It helps.