My heart aches for you, OP. You are in an awful position.
Please, as others have suggested, try to persuade your DS to return and live with you and his brother again (if he isn't still with you after his dad dumped him). Perhaps you could explain:
a) that you love him and miss him and are very worried about him (his school work, and ultimately his physical and mental health will suffer if this situation continues)
b) let him know that his father is unwell but will never get better until he learns to cope with things himself - this will only happen if he is made to take responsibility forces own life. Tell him it will actually be easier for his father if he doesn't have your son to worry about.
c) cut your ex off completely
The third is very easy from to say, but I now how difficult it must be. You feel you have to help your ex because your child is there - but if you can get your DS back, then you can be firmer with your ex., but even if your DS stays with his dad, please try to sever all contact. Change your phone number, don't respond to any letters/e-mails; call the police each and every time he tries to contact you (and keep a log) of any attempts he makes.
Your son will probably blame you for lots of things and for causing his dad's problems (I'm sure your ex has done his best to poison your son against you), and he will be very distressed and you ex will work on this. Stand firm, though it will be tremendously difficult.
You must be exhausted - the effort it must have taken you to come so far after so many years of being entrapped in a controlling marriage, must have been Herculean. And the effort and stress hasn't stopped yet. It will get easier, but I imagine that there are days you just want to sit and cry with sheer tiredness and despair.
Just remember - you have done the hard bit - you have stepped away. At the moment your ex knows that if he makes enough fuss then you will give in and do all the tedious little jobs he can't be arsed with. It is his way of retaining control. He is a grown man. You are NOT responsible for anything he does, so don't take that responsibility on.
I think he is unlikely to make a serious attempt to take his own life, though he may make a show of doing so to frighten you and your DCs. Ignore anything along these lines - remember it is manipulation. See if you can persuade your boys not to have contact with him for a month or so. He is obviously very manipulative. You have enough to do getting your life back and making a life for your boys.
If he says he is giving up on life/ can't go on/ has had enough, tell him that's up to him. He can do what he likes as it is no longer your business. And whatever happens (probably nothing serious) just remember IT IS NOT YOUR FAULT WHAT HE DOES.
Know I must sound like a hard-faced cow, but without taking really firm action you will never be rid of this pathetic apology for a man. he is using your emotions to continue to control your life. Please, don't let him when you have come so far.