The dog we are Pet-sitting is having trouble!?

We are pet sitting for our neighbors for a few days. The dog is very old, if I had to guess, he'd be about 15 or 16. When I went to let him outside, he couldn't use his hind legs. He was laying in his dog bed and I called for him. He used all his effort and his front paws were scratching at the floor trying to get himself up, but it was no use. His back legs just wouldn't move. I kept trying to make him do it on his own but after a while I gave up. I picked him up and tried to put him on the ground. He wobbled but didn't fall. I took him outside where he went #1 but he wouldn't move around (which he hadn't been doing before the problem anyways). I stayed with him for a while but eventually had to leave. My sister is taking care of him at nights and she just called and reported the same issues except that now he can't stand at all. He even peed under himself because he couldn't move. She said the same thing though, once he gets upright he's fine, but he can't get upright on his own. Because it's so late at night, we can't call a vet. Any help is appreciated! We'll take him to the vet tomorrow. Of course this happens while we are watching him! I'm very nervous about it! Please help!