I'm sorry @Tiredtrudy - your son isn't behaving well - but you asked whether you were being unreasonable.
Hindsight, of course is a wonderful thing, but as older and presumably wiser adults than your then 23 year old son, it was unreasonable of you (IMO) to accept your Son's offer of the loan. You couldn't afford the house you wanted and you should have stuck to your guns when you initially turned down the loan from him.
I am so sorry that you have got cancer and won't be able to work again and it is very difficult for all 3 of you, but regardless of what you gave your son before you had to give up work, you and he had an agreement that you would pay his loan bill of £400 every month and he would give you £300 'board' money. What you gave him as 'gifts' as his parent, is irrelevant , as is his salary
So it seems reasonable to me that since he was paying £400 towards the loan, it was equally reasonable that he offset the £300 due to you against the loan. That still left you owing him £100 every month.
You have paid him £3200 - but as the loan will also have included interest - you will need to work out what the total loan was for, less the £3200 and then also deduct the £300 a month he was due you for the unpaid board - and that final total that is what you need to pay him when your house is sold.
Of course your son is behaving very badly, but I nevertheless have some sympathy for him. I do wonder, from your posts, whether you have treated his worries about his finances, as important as they clearly are to him, or whether you have been a bit dismissive of initial requests from him to sort things out ( as you didn't have the money to give him then)? Is that a possibility? You say he has always been a sweet boy and his behaviour now is not like him. So it sounds to me like he is at the end of his tether and is feeling shafted by you and his dad?