Hey,
1st off, you have to stop of giving your money to him. Since he doesnt feel that he lost since its not his money.
2nd you need to have a serious talk with him. and putting all proof on the table of the amount that he owes you. I would give him 1 week, to come back with a plan of refunding you. Otherwise, you will feel oblige to contact his family member " gf/wife, or parents" for them to be aware of the situation.
Its like drugs/alcool problems, he is dependent on it and doesn't feel it yet.
If its a group of friends that provides him with the money, then make it a big meeting with everyone around the table. so that he can realize how much money he owes.
its sad to see a friend falling low, but u dont have to lose your own shirt also. Because those thousand of $$ are yours.
i would involve someone from his family right away. especially if he is still young. because u know, more time he continues more he fall deep in it.
good luck!