You obviously went through an awful time and it is admirable that your son stepped up like this, rather than leave you to it.
My brother feels a bit like this happened at home although from my perception it wasn't that way at all - and the circumstances were much less extreme so it is not comparable to your situation but certainly he seems to feel that way. He just wants some acknowledgement from our Mum and I think he would feel a lot better about it.
Possibly when your DS came home it was what he needed too: he must have been devastated by the loss and being able to step into his father's shoes may have given him a sense of pride. From a practical point of view, it sounds as though he continued his education or development and it is not unusual these days for people in their 20s to stay at home for this purpose. I am sure there were benefits for him as well as self-sacrifice.
It sounds as though over time you took him for granted and probably felt quite frightened to give him the freedom that might take him away from you through friends, relationships etc. which has tilted the balance and that is what you need to repair.
If your financial situation is ok now, can you make a gesture towards him to show your appreciation? A gift of something which relates to a passion or something he may have done if he hadn't taken the safer career route?
If not, then at least make sure he is no longer contributing to your life financially. Make time to see him and take him out for coffee or lunch. You aren't going to re-establish the parent-child relationship but you can find something new.
Be honest and tell him you have been thinking about how much you appreciate what he did and that you feel that you could have done more to make his life easier. Take care not to make it about you or ask him for reassurance. Just give him the reassurance and acknowledgement he may be needing.
If it goes wrong and you end up saying the wrong thing, or it opens the floodgates to him saying some things which may hurt, try to keep perspective, it may not all be sorted in one meeting or letter. Lots of small things over time have caused this, and lots more small things are what will make it right.