None of you come out of this well, OP.
Your son should not be shouting at you, but I can understand his frustration. He can't move on with his own life and buy his own home, because he (naively) took out a huge loan so that you could buy a house you couldn't afford. And it's put his relationship with his GF at risk, because she wants them to be able to live together in a home of their own, which is also understandable but not at all helpful.
Taking out a loan as well as a mortgage for a house purchase is a high-risk strategy, especially relatively late in life when you're probably at or near the top of your earning potential. Mortgage companies have affordability criteria for a reason and you encouraged your DS to borrow £20k because those criteria meant you couldn't afford the house you wanted.
I'm really sorry you developed significant health problems, but that's what happens as we get older. I had to reduce my working hours by 50% at 60 because of health problems that started in my 50s. Thankfully I hadn't committed myself to a costly house purchase and I'd been overpaying so that my mortgage was cleared early.
Get that house on the market asap and when it sells, clear the balance of the £20k loan. If you get a lump sum as part of your ill-health package before it's sold, clear as much of the balance of that £20k loan as you can. Make sure you're getting every penny you can in benefits, eg PIP and new-style ESA when you are no longer getting paid (get advice from CAB or other welfare rights service).
And try and build bridges with your son. It's absurd to try and count the cost of his education as part of this messy equation, I doubt if he had much say in it. And while it was kind of you to pay off his car loan, you were presumably comfortable with doing that at the time; plus you benefited from it in the short term as he probably wouldn't have been able to borrow the £20k for your house purchase if he'd still had the loan.