Hi, itm, I know this is so tough for you, with your S being so upset. When my S was 13, he and I were outside on the patio, laughing and having a good time, when he wanted to know why his F acted so protective of his car. He said let's go look, so, dummy me, not suspecting anything, went to the car with him, and found letters and cards and cologne gifts from ow to my H. So, he found out when I found out about ow. I wish S had never known about ow. He acts respectfully towards his F, but he hasn't regained all respect for him. Anyway, would it be possible for you and your H to sit down together with your S and let him know together that you both love him, and that you two are trying to decide the final steps, and that whether or not you will end up back together, that nothing will change the love you have for him? I don't know if DB advice is contrary to this, but children should come first at times like this.
Maybe you could ask H to please sit S down and talk to him. If his grades are suffering that badly, he needs counseling, too. Could you talk to his counselor at school? They are probably used to all sorts of situations, and are usually very kind, and look out for the best interests of the children. When I went to IC, he suggested I bring my S, too. So, I did, and S got a lot out of it.
I know what DB says about sending letters, but I can understand your wanting to just let him know how you feel. Save it. Maybe one day....