How can I make my fostered dog pee?

I am fostering a 2 year old German Shepherd that I found on the street and he just WON'T pee! He doesn't pee inside but he doesn't pee outside either. It's been 2 days and he has done nothing, not a drop! I am keeping him in an apartment and all he does is rest on the floor. He poops normally but he just licks the pee of other dogs and smells it constantly but will not raise his leg and go... Let me add that he is not castrated yet. We have already been to the vet and he had his shots and blood tests and no signs of disease. So what could I do?

Maybe you could give him some kind of water pill or some thing.