Senior dog can't get up anymore.

As of yesterday, my dog no longer had the ability to stand or walk. He is a 13-year-old rottweiler and boxer mix. Over the past months/weeks he's been getting more and more bony. He still eats and drinks with my assistance, and he tries to get up (often just spinning, but he did flip himself over on his other side last night) but can't. His back legs can move from what I've seen, but doesn't get up. I don't know if it's his hip or spine or whatever else. When his spine is touched, he kicks his leg. Can still turn his neck and head and stuff.

I guess my question is what do I do? I can't take him to the vet on my own, I don't have any money and live under my parents, and they don't have much money themselves, let alone any money that might be needed for him to have any surgeries. I don't know if he's in pain or not since he hasn't ever been very vocal.

Will he get better on his own? Should I let him go? How do I know if he's in pain? What do I do.