I totally agree!As the others have said, it would really help if you could tell us what food you're feeding him now. If he is on a urinary prescription food, there are still options if you're willing to try via diet.
You would need to start by taking him to the vet for a urinalysis and urine culture. Even though nothing was found before, it would help us to know what his urine pH is, and it would help you to ensure there are no crystals in his urine. Was any imaging ever done? I had a cat where nothing could be found in his urine. Yet a high resolution MRI found a small stone embedded in his bladder wall. Once it was surgically removed, the problem resolved.
I have a cat right now where an ultrasound found three stones in her bladder. That raises the urine pH, which then creates an excess formation of crystals and causes her pain and discomfort. We have had her on a urine acidifier for the past month. If that doesn't dissolve the crystals, she'll need surgery to remove them. But it's possible he's got stones in there that don't show up in a urinalysis, but will show up if imaging is done.