Just one of those things unfortunately, and I was actually talking about this last night with a friend who keeps being asked for money by someone he's known a long time.
I've always politely refused to lend money to anyone due to the fact it causes friction further down the line if you have to start chasing it, which I just can't be doing with as you can end up losing friends over it.
In fact I did give some money to a friend a few months back but i just told him to keep it as he was in genuine trouble.
In your case the £500 part was the loan so at least you got that back, but I would just write anything else off, lesson learned.
Just keep reminding him it's definitely his round in the boozer.