My first guess is that he is a male, and he is "spraying" not urinating. Had
that problem with a cat that was too young for the vets to sex. Was certain
it was a she. Then the spraying started.
If it is urine for sure, and not spraying, you might want to check the
litter. Is it clean? out of the way? is it a brand he likes? Cats are
finnicky.
Just some thoughts.
al