He has been emotionally abusing you for a long time, to the extent that you have completely accepted his nonsense accusations that you are to blame for everything, and that he is just a poor victim. It's quite sickening to read the way you write about him actually; the way you tie yourself in knots to blame yourself and excuse him.
I don't know whether he genuinely intends to leave you or whether it's just a manipulative tactic to force you into line. If he does intend to leave, it's almost certainly because of your fertility and nothing to do with your son or anything you've done or haven't done. Having a child with him would make you much more dependent on him, practically and financially, so now he knows that can't happen, he's lost that future power over you.
If he genuinely has mental health issues, they are his responsibility, and he needs to see his GP, get treatment in the form of medication and/or therapy (CBT or counselling). Of course he might not actually have mental health issues at all or he might have them but have no interest in resolving them, since they give him a very convenient excuse to treat you badly and blame you for it.
I advise you to read "Why does he do that?" by Lundy Bancroft and get some counselling for yourself (not couple's counselling with him) to talk about his behaviour, the relationship and the impact on you and your son. A good counsellor will help you work out for yourself what you want to do.
Do not get couple's counselling, it is never recommended with an abuser.