It has been my experence that cat's are prone to (UTI) urinary tract infections. This is caused by dry food crystalizing the urine and thus causing them to have problems.

another reason could be if you have changed his litter recently!

if you have other cats, he may not like to share.

there are several reasons this could be happening, like you I would go see the whitecoats for proper diagnosis.

Good Luck!!!!!!