If you're not ready, you're not ready - and that will have an impact on you after you have a child to care for. It will be difficult if you were 100% ready, but if you're not? Mentally, I would be concerned about your wellbeing.
It's really tough living with the uncertainty and loss your DH is facing with his condition, but the answer is for him to grieve and try to come to terms, not to pressure you to do something as huge as having a child before you're ready.
Besides which, there's presumably no guarantee that if you had a baby next year he wouldn't already have compromised mobility by the time that child was mobile. Rushing isn't the solution.
He imagined being a dad who could run around, but being a dad is so much more than that. I would be encouraging him to think about what's important to him about the role he has in his child's life beyond his own mobility, and how to have the kind of relationship and role that would be meaningful and important to him.
Because looking longer term, putting so much emphasis on needing to be mobile to be a "good dad" is setting him up for so much more grief and depression if/when he loses that but hasn't built up a more balanced internal view of all the ways he can be a wonderful dad regardless of his mobility or lack thereof.
Does he have a consultant for this condition who could make a referral to clinical health psychology services? I am not suggesting that because I think he's "crazy", but because they could really help him live more easily.
Health psychology is a separate specialism to "ordinary" mental health services in that it's specifically for helping people facing the kind of circumstances he is and helping them to still live in a way that feels valuable and meaningful - even if the picture of their life is different to the one they always imagined. It can still be a wonderful picture and they can help him create it, because it's probably quite hard for him to know how to redraw it on his own.
Health psychology services tend to be attached to hospitals and accessed via referrals from hospital teams, but you'd need to check how things are structured where you are. It's not the normal sevice where you go to GP and ask for a referral to counselling, so if you need to search or ask around make sure you state "clinical health psychology".
Ending up with some well meaning person sending him off to IAPT for a generic CBT course won't help in the same way (it would be a bit like being sent to see an ophthalmologist when you need a gastroenterologist), so you might need to do some research on services in your area and how to access them to make sure he gets to the right people.
Although it's totally understandable he would feel panic and sadness (and I'm sure lots of other things), the answer is support for him in finding ways to cope with the loss and unfairness he's feeling about the future he imagined possibly being denied him, not pressuring you to try to compensate and guarantee him a future nobody can guarantee.
Better for him - and his ability to live with the uncertainty of his diagnosis - to get that support now, before the stresses of children (and possibly his own physical deterioration) come along. Presumably this isn't the only area of his life affected by the diagnosis.