I think you may be writing the end of the story based upon your feelings and fears. And I also think you might be engaging in all or none thinking.
Let life work itself out. Think about what it would be like to have faith that the right thing will happen. Your son has made some good choices. There's no reason he cannot make more. He might.
Try to think about what is served in you by going to the darkest, most dire potential future.
From all you have written here it seems that your son and you and your husband are tightly bound together.
From my own experience, things seldom proceed in a constantly upward tangent. There are nearly always fits and starts. Are you interested in allowing life to work itself out?
How are you helping yourself and your son by jumping to the worst possible potential future that can be envisioned?
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There are a number of mothers on this site with only one child. I am one of them. I have said to some mothers with multiple children, that I envied them. I think this was foolish on my part. Every child occupies in us a space that is singularly them. Our hopes for them can't be replaced by another child. I think the pain is the same.