If I remember correctly this is not the first time he has sent almost the same email message about S17. I mean almost word for word at the start. I find his concern for how his S is doing in school less than heart warming. If he truly cared about him he would be making some kind of effort to rebuild a R with his S, not scold him about his grades, especially considering the pain and embarrassment H's choices have caused his S. He is a teenager that has to deal with everyone knowing what his father is doing, that he got some young chickie pregnant, left his family...that is A LOT for a teenager to have to handle. For a father of a 17 year old to just let his kid say F you and let it stop him from trying to rebuild trust with him is just so immature. For him to say "These kids need us and YOU have to do anything is just such a cop out! What is HE doing except looking at S17's grades and yelling at you to do something about it.
From what I see your H is a coward. He is afraid to make any effort at being a parent to his kids. He is afraid to admit that his actions have much to do with the problems his son is having in school. So what does he do? He tries to blame you and your "anger" for what he and his ACTIONS have caused! You don't owe him any explanation about what you plan on doing to help your son get his grades up and get through the mess his father has caused in what should have been his best year ever up to now..Sr. year.
You handled that email very well, better than I would have under similar circumstances for sure. Keep your head up Mighty. You are so strong and you will get through this and so will your kids...because they a have a mother as strong as you!