What a horrible father, who would do that to their own son, play nasty games?
Steel yourself and get tough. He is throwing his weight around because he’s been found out to be an asshole. His son will notice, maybe not now as he’s just confused, but all of this will go in. Note every single action of this down in a book and record it, calmly. Bring it up with a solicitor when you see them. Your son will come back at some point, so don’t get drawn in.
Time to keep handovers absolutely minimal, don’t agree to anything you don’t want, talk anything through with a solicitor first and ‘grey rock’ it all - which means act very boringly and don’t message if you can help it. If you do need to message him, keep it short, simple as if someone in court might read it.
You will ‘win’ in the end, as in, you have behaved like a decent person, who kept working on their marriage, didn’t cheat, didn’t lie, are not playing your kids to score points. Keep on doing that. Truth will out, it already has.