He's 15 and he doesn't use the litter box at all. We clean it out every day or so. I should say there is only one box and two cats. The other cat is younger and doesn't mind using the box.Is it because he is older he doesn't wanna use at all?
You really need another litter box. The rule is one box/cat + one more. Also, is the litter scented? Cats don't like scented litter boxes. Is the box hooded? Cats don't typically like hooded litter boxes because it not only traps in odors, but also cuts down their ability to move around as they please in the box(most cats will stay on the rim as much as possible so they're not stepping in anything).
What type is the litter? Clay can be irritating to their paws. Especially older cats who might be having a hard time cleaning it from their paws.
The first thing to do is to take him to the vet. The number one reason a cat avoids the litter box is illness, and at his age it could be any number of things. Make sure the vet does blood work and a urinalysis.
In addition, as the other poster said, add a couple of litter boxes, it may be he doesn't like to share a box. Add some low sided uncovered litter boxes that are easy for him to get into, in case he has arthritis.
But please do take your senior cat to the vet to rule out things like kidney disease and diabetes. The sooner a disease is caught, the better the outcome.