I feel with you, jhr1131, silent treatment is not easy to deal with!
I see 2 topics in your post. One is the triangle between you, your son and your h/his father. Your son is a young adult, so he has to find his way with his father. So when he feels the need to send this letter, I would only advise perhaps to sleep one night over it and recommend clear I-statements.
Other issue is you, undecided, with silent treatment and accused to have an affair. Do you know this workshop?
BPD BEHAVIORS: Silent treatment - verbal abuse..