I'm so sorry. IME many men decide they 'can't do this' and there is a better life deal for them somewhere else when children turn out to be disabled.
My exH simply denied that the disabled children he fathered with me and later his other wives, where his! It didn't fit his self image, so they couldn't be his.
Many years later I found love for the 1st time. (marriage was arranged)
Ex partner actually had a breakdown and was a missing person at the point our Ds who was 'different' but fine at home, started to show serious differences and difficulties at school. Not sure ex could have picked a worse time.
It was only then that I started to discover that while everyone thought he was a lovely person, a perfect Dad, and no way would he have done this to us, that his family was hiding a whole story about his 'differences.'
Frankly him getting together with me was them safely seeing him offed onto a partner. It was only after he'd been missing over a year, they gave up asking me what I'd done, and their facade about the whole family started to crack. Many 'oddballs' but a close recent relative with a similar profile had broken, and killed his mother, so covering up (ASD) symptoms had become a huge deal.
Ex turned out to be covering ASD himself, but he no idea that was what 'it' was, just that he mustn't be like so and so. He'd been taught to mask who he was, what he really felt and thought, and cover the 'shame' of the reality because everyone was terrified of the (ASD) line down the family.
He 24/7 masked to everyone all the time in order to survive and appear 'normal' until the day he couldn't anymore. His Ds was turning out like him.
When he was finally found, he was unsure who I was, and he'd blanked out even having had a child.
Ds's profile was different to your Ds, but as a child I was told it was utterly unrealistic to expect that he would ever achieve a career and successful independent living.
While I doubt your husband has actually planned it all this way, It's also no surprise this has erupted at a point you are particularly vulnerable and dependent on him.
A) He's ashamed of his son, but it was just doable while he could hide behind the rock- you. But his 'rock' has been shaken, you're sick, you're fallible, you turn out not to always be able to make it all work.
B) he's had to face the situation more and seen how little he knows and placed that on you, (there may be some reality there in terms of you allowing his detachment) and translated it as you 'let' Ds 'become' this.
C) subconsciously (hopefully only!) he knows you're in a weak position to fight back right now.
It is possible it's just an emotional reaction to finding out what life's like when he has to do part of what you do automatically, and discovering that 'disciplining it out of Ds' really doesn't work and has decided that's because he's 'been allowed for so long' and he's just verbalizing his feelings. But the chances are it isn't, and if this is not exactly now, then it will be later. The genie is out of the bottle that he's beyond not happy with the situation, and importantly, feels he has the right and ability to have a better life, free from the one he's made.
On a practical note, be they arseholes or fools, I'm afraid IME the offers they make at the beginning of extracting themselves are the best ones they can imagine that make them feel they are being fair and reasonable in their eyes.
These get lowered, redrawn, and withdrawn as actual separation becomes a reality and they start seeing how much their new shiny life will actually cost to achieve and maintain. I'm telling you this because right now you're in shock and unlikely to be thinking about sealing anything offered. They don't generally up their 'deals.'
Very, very, different situation and different realities for fathers, but I gave D's dad the option to be a walking wallet and financially deal with authorities etc, (his nightmare) or I'd forgo maintenance if he accepted learning from me how to Co-parent his ASD D's from his new separate life and be there as his dad warts and all.
Obviously financially it was disastrous for me, and stripped my career and life, but having an ASD father who'd masked for the world and been brought up in denial only to later suffer serious burnout, in the picture as Ds grew up, helped me help him shape into understanding himself and developing into a man. It also meant when education failed, having done everything else myself, taking on successfully home educating him, was less daunting.
They kind of grew up together tbh, and get on well now, and while I wouldn't recommend the way it was achieved, I don't think I could have created anything better in terms of giving my Ds his best crack at developing well regardless of the ND issues he has had to learn how to manage. Keeping his reticent father in the picture at any cost even though at arms length, was the right thing to have done for him. He was 14 before I started being able to leave them together alone at all. there have been some dangerous moments for all.
It's been very spiky development and we're not quite there yet but I can see the feasibility. If you'd told me we'd be anywhere near where we are when he was 9, I'd have ignored it as impossible.
A hell of a price has been paid, and there will be smarter ways nowadays, but ultimately getting my Ds to (almost) independence and knowing I've done enough that he's fairly equipped to be able to make it out there after I'm gone, is the thing I actually wanted the most once I got past everything else.
But, I only knew that was what mattered most to me, after everything I thought mattered was gone.
I can't tell you what you should do, but I wish you every strength on top of what you clearly already have.