I'm sorry to hear about your situation. Here are my thoughts.
It doesn't sound like H is doing much for S these days. I suggest you consider requesting 100% physical AND legal custody. H has checked out of your marriage and removed himself as a decision-maker as to S and you should confirm that legally.
Don't write H a long letter. That would be a waste of time. Instead, consider preparing a short list of the things S needs from H going forward. As part of the list, you can explain what S is going through, because it is important for H to know that. But as for your thoughts and feelings about all this, who cares? You, but definitely not H. So, save all that for this board, your therapist, and the one or two close friends you know you can count on.
Reaching back to one of your earlier questions, I agree that when it comes time to file something (legal separation or dissolution) you should be the one to file and that you should file first and then tell H you did it so he knows the papers are coming. You can soften the situation by asking him how he'd like to receive the papers (just make sure you legally serve him one way or another).
But, BEFORE you do any of these things, please talk to an attorney! Don't just rely on the Self Service Center to get you through this mess. At the very least, for your sake and for S's sake, spend a little money to talk to an attorney first.
There may not be much (or any!) money or property to divide from your marriage, but S is priceless and it's worth it to part with a few hundred bucks now to get some competent advice about the following:
(1) Legal separation vs. marriage dissolution as the initial move;
(2) How various choices may affect your H's future ability to retain residency in the US;
(3) Steps you should take now to protect yourself and S against problems, allegations, and threats that may crop up during the divorce case and afterwards; and
(4) Things you can do to minimize the stress on S.
I'm sure you have a friend or two who can recommend a family law attorney, so start asking around. There could be many things you haven't even thought of, and if you don't deal with them now, they could come back to haunt you later, years later.
