Why do you feel your H is trying to reach out to you? IMO this is nothing more than blame shifting. When your H lives with his pregnant mistress he doesn't get more than child related talk. I am not sure why you told him he was being standoffish. What did you hope to accomplish by telling him that?
IMO he knows you feel that way now so it was very easy to turn it around on you. It is his way of trying to bring the dynamic back to where HE is comfortable and he is comfortable when you don't change.
I would mail him the totals of what he owes, let him know he has to pay it by a certain date and ask him what his solutions are for future letters that will be sent to BOTH of you regarding son.
And if your H calls and is "furious" simply tell him you understand he is upset but you choose not to converse with him unless he is calm.
