I'm astonished at some of these responses.
The OP bought a house well above her means with her DH well above an age where normal people would consider getting out a mortgage due to being close to retirement age. In order to do this her DS who was 23 at the time took out a loan and gave them 20k.
The agreement was that he would move in with them in return for taking out the loan in order to help him save for himself to get a deposit to get on the housing ladder, OP was supposed to make the loan repayments.
However OP hasn't been making loan payments, her DS has been making loan payments because unfortunately OP became unwell and hasn't been able to work (which is another red flag for not being able to afford the property in the first place if it was unaffordable on one wage).
Whilst it's not great how your DS is behaving I do think you're behaving appallingly. The loan will have come to more than 20k with interest taken into account, this will be on his credit report and he's unlikely to be able to get a mortgage whilst this loan is outstanding. It's possible he might not pass credit checks for renting either whilst it's outstanding. He's probably worried about this hanging over his head, can see your finances are shit and has realised that by helping you he's thoroughly screwed his chances to get on the housing ladder himself for another few years.
Should he be paying board/food? Yes, but equally you're only in your home because your son not only took out a loan for 20k, but has been paying that loan because you haven't and he's been forced to pay it because if he had relied on you keeping your word his credit would be trashed.
Top it off you've thrown at him all the prior finances you've covered for him in the past including private school as though he should be grateful that you did all these things and he owes you for it (entirely your choice to send him to private school and should never be used as a weapon to beat your child into being grateful or expecting repayment).
Don't be surprised if your son never does another favour for you again. He was unwise to do it in the first place and probably never thought for a minute you wouldn't make the repayments or throw everything in his face that you've ever done for him.
Sell the house, it's unaffordable and pay your son back, not just the 20K, but the full loan amount plus the interest and any early redemption fee that he will owe the bank/loan company for paying it back early.
You're in real danger here of not doing real damage to any future relationship with your son.