I am shocked by some of the awful things said in this thread. Well done OP for keeping going. Your son's behaviour is abusive and I am not surprised you find it upsetting. If I were you I would sit him down and make him listen to/watch the recording and tell him he needs to leave unless he agrees to stop. If this is out of character there could be something else going on and if so he will hopefully share this with you. But. either way, you do not deserve this at all.
I can absolutely see why you mention the financial help he's had and that he has had a privileged upbringing. You are not saying that these monies should offset the loan, just demonstrating that he has been extremely well catered for and has plenty of money despite the outstanding loan.
As for the money, well I cannot see that you owe him anything if I have my maths correct. The agreement was that the loan would be repaid over 4 years.
In terms of what you should have paid on the loan to date, this is 21 x £400 = £8,400. You have only paid £3,200 so you owe him £5,200 for the missed payments he has covered.
However, he owes you 21 x £300 = £6,300 none of which he has paid. So he in fact owes you £1,100. I get that he has paid the loan off early but that was his choice and not part of the agreement. Despite that, if you had the money to help him now I have no doubt you would do it. But you don't and well, he will just jolly well have to wait won't he. The £1,100 is more than enough to cover what you owe him each month as you wait for the house to sell, as you only technically owe him £100 each month he is living at home.