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.
Declawing: please read this first
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