I have read this, and it really resonated with me, as i watched my parent's marriage disintegrate into a sexless, loveless marriage as i grew up. My parents stayed together for the sake of the family and home, but I know how desperately sad and lonely it made my dad over the years. We did not have a sibling with SN though. Also my mum seemed bitter about the life she had found herself in and almost used sex and affection as a power game as she withdrew from the relationship with my father; whereas your wife has seemingly stepped back from her emotional life as a consequence of the stress and strain of having a poorly child, so the reasons are more understandable.
However, the outcome is not that different... my parents were married for 40 years, with probably 30 of them being sexless, sleeping in different bedrooms etc. Neither of them were happy, and us as children were aware of it and at times actively encouraged them to separate. Sometimes they hardly spoke to each other, other times they had ferocious arguments, but then they also they did on occasion seem to enjoy each others company and our family life. But as far as i was aware, my father craved having a proper relationship with my mum, whilst he did what he thought was the honourable thing and stayed and was in fact our primary carer a lot of the time. She however, seemed incapable of really allowing herself to love him or be intimate with him. Whilst I believe my mum initiated the problems in their relationship for reasons I don't think I'll ever truly understand, in the final years of their marriage I saw my father start to change... after all the loveless years together, I think he snapped and the resentment built until he drank too much, made snidey comments, was impatient and inconsiderate to my mum, lost his job, forced my mum to be the primary bread winner and was even cruel at times to me with the things he said. I actually felt sorry for my mum and struggled with this change in him. In fact once we had all left home, I honestly think they hated each other but felt too old to start again, and I hated watching it so visited less frequently.
Then my dad got ill... and during his illness, i was so surprised but my parents found each other again. My mum cared for him, they talked for hours, they laughed and cried together, held his hand, kissed... it was so beautiful and tragic at the same time. When he died, I think they were more in love than they'd ever been. I'm sharing this with you, as I guess what I'm trying to say that you need to make some big decisions before its too late, but that it is possible to repair a relationship like yours. Just don't leave it until you don't have time to enjoy your life together again.
Your wife as other people have highlighted, needs to find a way of dealing with her grief around your son. I think therapy and potentially antidepressants might be a good start. But in the meantime, I think you need to leave or decide that you will no longer allow yourself to feel bitter and instead be positive and work hard to try and rebuild your friendship... maybe a way to start would be to do things outside of your usual responsibilities, to try and brighten up her day. Maybe a bunch of flowers, vouchers for a spa day, a lie in, breakfast in bed; share funny stories about your day, leave notes around the house telling her how much you appreciate her and everything she does for your family, buy her favourite biscuits or wine, tell her about a boxset / tv show you'd like to watch with her so you have something to talk about etc. Do all this without expecting anything back. And don't stop. Be persistent, even if it feels futile in the beginning. I think slowly but surely she might actually start to look forward to reading your notes, the massage you've booked for her, the book you've bought her to read because you think she might like it etc. Think small gestures, not big over the top things which will make her feel pressurised. Try to bring some joy and happiness into the home. Would a pet be conceivable around your son? I have found animals a wonderful way to bring light into a sad home. If you want to save your marriage, then you are going to have to fight for it. But if you find your friendship again, I think your wife might start to engage with you again, and hopefully it might bring some affection back to your relationship. Patience will be needed, but you've been patient for 14 years, so you know you can wait a bit longer for the sake of rebuilding your marriage one step at a time.