My problem is of a legal nature....?

First thing's first: go speak with your lawyer and file a suit against him. Not only for breaking the separation agreement, but also for withholding child support payments. You absolutely have a case with both of them, and he will be ordered to pay you, no matter what. If you want to sue the ox off of him, I suggest you speak with your lawyer about that also, to get reparations the divorce agreement did not afford you, if that is possible. He can advise you of your legal options for that.