My ex has distanced himself from our sons all their lives, because our first child was stillborn, and he's terrified to love the surviving children so it won't hurt as bad if something happens to them. Sounds like your guy is doing the same sort of thing. All I can suggest is have him talk more in-depth with the doctor, in the hope that the doctor can reassure him that asthma is not necessarily a fatal disease, and can very easily be managed. It might also help if you can help him to see that loving and losing, as painful as it is, is still infinitely better than never loving at all. Not only that, but he's gonna love the little guy no matter how hard he tries to hold himself apart, so he may as well go the whole hog and let himself enjoy his son.
My ex has missed out on a lot of wonderful moments with my sons because of his distancing himself, moments that myself and my new husband are enjoying immensely. Good luck with helping your hubby to see what a waste it would be to miss out on all the cool stuff a child can bring to his life.