Your h is rebelling against you and the rules concerning the child support. He's going to show you and everyone else that he's going to do it his way. I think I would not mention it again for a while that he should be writing the check to you. If he sees you and your son aren't reacting to his behavior, he may just cease it.
As for your son's behavioral problems...sounds like he's the one that is projecting. I think you are handling this situation w/a lot of patience.
Now, about answering the door, I would advise him the next time he appears at the door, that if he attempts to run roughshod over the person answering the door, then you will have no option but to tell him to mail the check to your son by certified mail, return receipt. You can also point out that you don't show up at his place unannounced and push your way into his space and you would hope that he would do the same at your place. If he continues to act this way, don't answer the door or look out the window, see who is there and go out another door and meet him in the yard...but doing what he's doing is not acceptable.
Your xh is not happy and he doesn't want to see his family happy and yes, he's trying to convince himself that he did the right thing by walking out, thus the fights ensuing.
Try to dig deeper for patience and set up stronger boundaries and adhere to them.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
