Often times when cats don't go in their litter box it is due to some environmental change. Depending on the cat some can be very sensitive to changes. Has anything in your house changed since this started happening? Did you introduce a new pet, lose an old one? Have you been under a lot of stress lately, has your schedule changed?
These are some possible causes that I have seen affect cat's bathroom behavior. And of course make sure that his litter box is clean. Cats much prefer to use a clean box over a dirty one. I am not sure if there is an underlying medical problem, but certainly have him checked out by your vet if you don't feel that it is environmentally related.
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Nick
Vet student
