You're in a sticky situation. Not blaming - but lending someone that amount of money with no ways of paying it back was not a good idea (that's why bank need to see assets or regular income before they will loan).
Just out of interest - what did he spend the money on?
You have proof that it was a loan so that's not an issue. However at £65 a month that's over 2 decades of payments. First option is that you count this as a blessing (you will eventually get it back) and write out a contract now and get him to sign. It needs to acknowledge the total debt and the 23 years of payments, plus interest (maybe RPI?)
You could take him to court - he's acknowledged the loan, however as he has no tangible assets, they will find in your favour but he's not going to be able to pay you.
Legal advice is always good, especially if you can get the first hour for free, however advice/letters etc are eventually going to cost you money and may not change the situation.
Despite what some others have mentioned, a court cannot make someone take out a loan in order to pay you back (a court can't order someone to put themselves in debt, they can only order them to free up fixed assets).
My advice is to speak to his family and make them aware - can he borrow (some) all of the money back from them?
You said yourself you think you already know the answer to the question......unfortunately it's a very expensive lesson.
Don't lend money, that's what banks are for.