WR, I think your h is a lot like Bea's h. He can't let you go and he doesn't care what he has to do to keep you wrapped up in his drama.
I would have a nice chat w/my lawyer and I would advise him/her, that enough is enough and let's get this stuff in writing so that he doesn't continue to come back w/the same BS. All he thinks he's doing is paying you for support, not for his sons. Your sons don't fit into his sick equation at all.
I don't think I could be upset by this latest stuff...more like I would be furious w/him. You've tried being pleasant and civil w/him and yet, he still is snapping at your feet. More distance is required on this front and this may explain his comments about seeing his son whenever his son wants to and also he would get first dibs on his son's time.
I do understand your position and that's why you need to call h is bluff on this and go to court. It might make he realize that he can't control you or your life any longer. One quick question, why wasn't this letter sent to your lawyer or did your lawyer receive a copy of the letter too?
In the meantime, please try to take care of yourself. Monday will be here before you know it and then you'll have some answers then.
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.