Hello and welcome!
I'm sorry Diamonte is experiencing these problems. Typically, the surgery (a perineal urethrostomy) is necessary when a cat keeps developing blockages and the only way to eradicate this, is to remove the most narrow part of the urethra (located in the penis) and attach the bladder opening directly to the opening leading to the outside of the body. By widening the opening, this decreases the chances for urinary blockages. It doesn't remove the chance for urinary tract infections (UTI), but you wouldn't have to worry about future urethral blockages.
The bladder/urethra can become blocked with stones that can be caused by 'crystals,' sand-like particles that occur in urine which is not acidic enough. When the crystals clump together (and they have sharp edges, which cause pain/irritation), they form a 'plug' not allowing a cat to urinate. Stones can be formed in the bladder depending on what kind of crystals are present in the urine; there are two types: struvite and calcium oxalate. Here is more information: CLICK
Some male cats are more prone to blockages than others. As you know, this is a life-threatening condition. Even if medications and diet are tried, but, are unsuccessful, more than once, then, the surgery is recommended as a way to prevent future blockages.
It's best to feed cats only wet food; it's healthier and because it contains more moisture than dry, it can help reduce the incidence of blockages. Science Diet makes a food called s/d, which prevents crystal formation and also dissolves existing crystals.
Royal Canin also makes a food called S/O, which does the same thing. I would try to discontinue the dry food altogether, as this is a main contributory factor to male urinary blockages, and stick with the canned food. If you want to leave a little dry of the prescription food to help with crystals, but make Diamonte's main diet consist of the wet prescription food, that's fine.
Grain-free foods are great, and some well recommended ones are Wellness, Wellness CORE, EVO, Blue and many others, found in your pet supply store. However, these foods have to have a good 'urinary pH,' so the urine is kept more acidic. Wellness CORE and EVO have good urinary pH numbers. However, I'd try the Hills or Royal Canin canned prescription foods for this purpose, first.
If he's still visiting the litterbox very frequently and only voiding in small amounts, it sounds like the inside of his bladder and/or urethra is inflamed, irritated, or he could be bothered by crystals, again.
If the c/d is causing constipation, add water to the canned food, add 1 TBS plain, canned pumpkin (not pie filling) to a small amount of the food, then add the rest of the food, speak to your vet and ask if you can give a small amount Miralax or Metamucil, daily.
I'm glad to hear that he's drinking water, because the more water, the more the bladder/urethra gets flushed out.
It would be a good idea to have him seen by the vet again and determine through a urinalysis and culture, if he is on his way to being blocked again, or if he has a UTI or if he may have interstitial cystitis, an inflammation of the tissues of the bladder that cause the same symptoms of a UTI, but there's no infection, yet, it's painful.
I hope this helped and Diamonte does not need the "PU" surgery, but if he does, it will prevent future blockages, if he seems to be prone to them. You can also get a second opinion from another local, experienced vet, in person, if you're not sure if he should have the surgery, if it is suggested. While you don't want to rush into anything, you also can't have him be uncomfortable all the time and deal with recurring blockages.
I hope all will be well with your sweet Diamonte!
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