Well, yes he's perfectly within his rights to decide he doesn't want another child, but you are definitely not being unreasonable here - I'm just going to echo what you said back to you:
You guys had discussed it before having children, discussed it AGAIN after DC was diagnosed and agreed plans aren't changed, put clothes aside, made an appointment with a genetic counsellor to discuss your options for pregnancy, and within that appointment discussed being referred for IVF.
Throughout all this, your DH hasn't indicated once that he might not be on board with having a second child - he waited until after you had discussed the IVF referral to voice those feelings. So OF COURSE you are blindsided, and OF COURSE you are upset. He has no right to tell you not to cry. I notice he also said it had never been discussed - but you felt it had been discussed? And I mean, it had been discussed to the point that you both ended up in a genetic counselling appointment looking at pregnancy options, so...!!
I also notice he said it was a big decision and he couldn't be expected to make it quickly. Does that then mean that he might still want a second child, but just needs to take some time to think it through? Did he indicate what was putting him off or worrying him about the idea of having two?
It sounds to me like the two of you need to try to sit down and talk through your feelings about this so that you can better understand one another's viewpoints. Maybe agree to each other that to begin with you can each have your say, without interruption, and that the other person needs to just listen.
Best of luck x