I think it is a nice gesture that you lend employees money. My boss has also done that before, but I guess when it becomes a regular thing and when it starts bothering you you should definitely stop it without thinking too much of it.
Just explain your employee the situation and probably show him some other options that he has in
borrowing money e.g. banks, personal lending, social lending institutions etc. Doing that he will notice that you care about his financial situationn but simply can't keep on lending him money personally.
I bet he would understand!!