Al in hospital can't go home cuz his sodium is too low. Could his existing cronic kidney failure &stage 3 bone cancer contribute to that condition?

Yes. The most common diagnosis in advanced cancer patients is syndrome of inappropriate antidiuretic hormone. This often results in low sodium. Kidney failure can also contribute to low sodium levels.
Yes. Cancer causes something called SIADH making the body retain more water than salt leading to lack of equilibrium between the two. The test will show low salt, note it is actually more water and not low salt content of the body. That is why he will be told to cut on fluid intake. Ckd makes it a bit more likely for low sodium concentration to be severe.