ok it could be a numbr of things:
Bladder infection
Kidney Disease
OR
if you are feeding him poor quality food it can cause a blockage in his urinary tract which is not only painful but dangerous. (it can make him feel like he has to pee all the time) I know two people who had this happen to thier animals 4 years apart, in each case the vet did not come out and directly say it, he hinted that the fact they fed thier cats the cheap food and only hard food quite possibly was a contributing factor.
it cost them around 300-400 to get the animal back to normal.
I do not know what you feed your cat, but I highly suggest that you switch to a high quality food such as science diet, and 1/2 can of soft food per day the reason is that cats originated in the desert. Due to that thier bodies still operate on the need for little water so they do not drink water often, that is why thier urine smells so bad, it is because it is so concentrated. It is for all purposes it is amonia. (which by the way, NEVER use bleach on say like a concrete basement floor or garage to clean up cat piss. amonia+bleach=cloud of death.)
Because of this they must have another source of water other than the just the water bowl because they only drink when they are thirsty, that is where moist food comes into play, 1/4 can to 1/2 can in the morning once a day is enough.
I had a diabetic cat for about 12 years and one that is in the begining phase of renal shut down (he is 15) so I got to know a lot about thier physiology and needs from talking to my vet about proper care.