If he'd kept up his $50 a month he'd have paid you off years ago, so it can't be a misunderstanding. So he's either a thief, or more likely just totally disorganised and self-centred.

Of course the trouble with this kind of thing is if the person owing the money says you'd told him it was a gift or something like that, and you don't have anything in writing.

If you were in England I'd advise you to go along to the Citizen's Advice Bureau to get the legal position straight in your head before you do anything.