I'm sorry but your adult son isn't responsible for you living beyond your means.
It seems reasonable that he wants his money back so he can move out.
If you can't afford to repay him, then yes he is within his rights to force a sale and to make you live within your means rather than him being trapped indefinitely living with you.
You put your financial considerations before his and this has had implications for your relationship with him.
I'm sorry but your attitude about how your decision to privately educated him - and how he should take this 'off your bill' shows you are financially abusing him not the other way around.
He can't move out because you changed the goal posts and reneged on your agreement. If he wasn't your son, your mortgage lender would have pulled the plug had you been living within your means. Instead you manipulated him into lending you the money you couldn't afford in the first place.
He had a financial plan that you screwed because you wouldn't live within your own means - your illness was after this.
I'm sorry but his request for a weekly repayment plan / sale of the house is reasonable. This is what a mortgage lender would do.
You are emotionally abusing him by using the fact that you are ill, to refuse to go along with this. And then you use arguments about your financial decision to educate him to justify it.
It's clear you have no intention of repaying him. And you wonder why he's angry?
You've screwed him over to the tune of £20k which was the basis of his house deposit and his future.
You should sell. And live within your means. He is not financially responsible for housing you and again, suggesting otherwise, regardless of your illness/ disability is financial and emotional abuse of your own child.
Not cool OP.