I have two thoughts on your posting. 1) I would speak to your son's therapist about the letter and see what he suggests since he is working w/your son. He may be able to give you some insight as to what to do; and 2) If your son is insistent about writing the letter, then let him write it and put it in an envelope for a bit later. He may change his mind about giving the letter to his father after the holidays.
Your s is a young man who is trying to understand why his father is the way he is. He is doing what we all have done or are doing trying to make our spouses see the light. Because your h's interaction w/the children may be different from the interactions w/you, your h may respond a bit differently and because your s is 13, he may have some say in to how his visitations may go w/your h...it all depends on the area that you live in.
Talk to the therapist and see what he suggests. I know you are concerned about the custody issues, but your right now, your son's peace of mind needs to be addressed in the proper manner, i.e., be it writing the letter and giving it to his father or holding on to the letter until after the holidays.
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.
