I really do feel for you @Ciaocatnip.
We’ve gone through a similar struggle in our family. DC was born with a small physical disability and when it was investigated, it turned out they have a chromosomal abnormality inherited from me. It’s so rare that no one can tell us what, if any, future effects it will have. DH in particular was keen to have a second child and I vacillated back and forth - it was so stressful and upsetting when DC was tiny, knowing there was something wrong but not what, exactly, and all the tests and appointments. My health, both physical and mental, massively suffered during my pregnancy and afterwards. We saved all DC’s clothes and talked seriously about having another but I was never sure whether I could face all that again. We did try in a desultory fashion to conceive again and when it didn’t happen, we were referred for further investigation and possible IVF with PGT, but it turned out I was in very premature menopause and for all practical purposes, no longer fertile. It was very upsetting for us both but at the same time, a little part of me was relieved. We then considered egg donation, but eventually I decided against it for both ethical reasons and practical reasons - I wasn’t sure I could put myself through pregnancy again for someone else’s child. But even 7 years later, part of me yearns for the family we’d planned before having DC - two lovely healthy children.
Anyway, I’m merailing with a massive tl:dr. My point is, your DH may very well feel, like I did, that he wanted another child and so went along with discussions about bigger houses and saving clothes, wistfully wanting the future he’d envisioned but part of him not committed to the idea because of the reality of your existing child’s condition. And then when it came to the crunch, he’s realised he can’t go through with it. He’s not necessarily gaslighting you - he might just be feeling that he’s drifted along with the plans without ever consciously deciding to do it. Saying no now doesn’t mean he wants to have child with someone else, or that he is lying now or was lying then. It’s more likely his feelings have coalesced once the pint of no return approached.