That would be really tough. My oldest was born with a fatal disease. I can relate to a lot of what you're saying. People deal with grief differently. It was hard at first with my hubby and I. I felt lonely about it because he dealt with it very inwardly. If stressful things happened, he would pull in to himself, while I was needing to face this WITH him. I needed that comfort and communication from him.
But he was never in denial about it like you are dealing with. That is part of the grief cycle. Many people start off in denial. Then they'll move to the next stage. They might feel angry or accepting or emotional, etc.
I think I would suggest giving him some time. The grief cycle really is that - a cycle that repeats itself. Over a period of time, it goes through the cycle of different emotions and dealing with it, then it restarts. I've never had denial like your hubby. But I have had times where I don't focus so much on the reality of it. Then it'll suddenly hit me, and the grief is really strong. I think if there is truly enough evidence that your son has this illness, your husband will eventually come around to it.
If you need to go to counseling to figure out how to deal with him, definitely do. I'm guessing he won't go with you? My hubby and I haven't gone to counseling, but it's something I think would be very helpful for people in shoes like yours and mine. We would have gone by now, but our daughter's illness makes it difficult to have others be around here (she's immune compromised).
So many people get divorced over things like this, and I think it's because there can be such a difference in dealing with emotions over it. And, while one might be feeling withdrawn about it, the other NEEDS a connection with the other one. My hubby and I have talked about it and how we each feel. I've told him I need him to hold my hand at scary times and stay close to me. He's very calming to me. But I want to do that for him too. Not just about me.
Sorry, it's really late here. I think I've totally rambled. While he's in denial, I'm not sure what you can do other than wait it out. Or maybe have a doctor talk to him and explain the facts. Once he switches to a different part of the grief cycle, then I think you'll be able to discuss things more and share how each of you feel and things that you need from one another while you each deal with it. Because even though it's normal for people to deal with grief differently...you have to still take care of each other through it. And it sounds like you're feeling pretty alone with it!