In all honesty, he IS validating your reason for wanting to stay home. Your reason, if I understood you correctly, is that you were a SINGLE MOM when your son was born, so you want to stay at home for the first year of your next child's life.
He's telling you that you WON'T be a single mom this time around (because he intends to stay), so no need to stay home. I sort of get that.
Yes, you should NOT get pregnant while you are not on the same page with this, or it will be heart breaking. You have to resolve this so that you are BOTH ok with the outcome.
What will you do for health insurance if you quit your job? What are the medical costs associated with your son's special needs? How will you pay for the medical costs of a newborn?
As far as the monetary sacrifices..... Can you eliminate your salary NOW, before you even get pregnant? That would enable you all to see exactly what the financial changes would be now, before you add the stress of IVF and a newborn into the mix. You could also SAVE all that money - just have your entire paycheck put in right into a separate savings account at the bank via direct deposit so you don't even see it each month.... and then you would have that to fall back on if something happened and you needed the money when you didn't work.
Are you willing to compromise? Your post sounds like you're presenting an all or nothing deal, which didn't leave room for discussion...... just "this is what I want" and then his only response choices were "yes" or "no" - there wasn't room for discussion. Could you strike a compromise and stay home for 6 months?
And I guess finally, the question to ask....... how bad do you want a 2nd child and do you want one bad enough to go back to work after your maternity leave?
I guess if it were me, I would not talk about having a baby for a set amount of time - I'd probably give myself 3 months. Not talk. No plans. No discussion about work. If he brings it up, say you heard his argument but this is something that was really important to you and so you are weighing if you still want to have another baby if your "vision" of what you want for the first year is so dramatically different from his. Then set a date to TALK about what each of you want for this baby and for your relationship and for each other. He may not want a stay at home mom, because someone he knew was miserable / had an affair / the expectations for him will triple / whatever....... it STARTS with his fear of financial stuff, but there is always MUCH more behind that. What is HIS perspective? What does HE want for his child?
And then, yes.... do a budget. How much do you make? How much is child care? etc etc etc.
Good luck and I wish you all peace in this journey.