I'm sorry that you're so ill. It doesn't sound like you'd be fit for any kind of work right now.
However, I'm afraid your illness is where my sympathy ends.
You've had the talent or luck in life to achieve a high income as a family, but you seem to lack wisdom to go with it. I think this is part of the reason why you thought 20k was a wise or reasonable amount to borrow from your son, whilst most people would do anything they can to avoid that because they recognise it as a lot of money.
If you were able to send him to private school (without a bursary presumably), then I'm not sure how you've ended up taking a loan from your son who's only in his mid 20s, or why that was necessary. You say it was to do with age, but what it really necessary to have a house that cost that much extra, and ending up taking a large loan from your son, rather than settling for a bit less? Living out of your means comes to mind.
He had mostly a private education and I paid off his sports car finance two years ago. He's never offered to take that off 'the bill'
I know that might be a hard pill to swallow, but what you gave, you gave willingly as a parent. It shouldn't not have been given if you had high expectations of claiming it back off him later in life, or if so, that should have been made clear to him at the time.
My DH has told him I'm ill and when the house is sold he can have his money. It doesn't seem to be enough
Why is that not enough? Your son might be angry, but it seems like there's missing detail here. The fact that he's so angry suggests to me that he doesn't believe you will sell the house for some reason, or that it will take a very long time to do so. Have you already put it onto the market?
My husband things he's a privileged brat
You need to look at yourselves first and take responsibility for how you raised him before laying that on him as some kind of abstract personality trait.
I don't agree with the shouting, but if you want your son to believe you're going to downsize, you need to get going with that ASAP.