Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
First, I have to say that these cases are truly challenging for the cavy, their owner, and their vet. This is because there are frustratingly no specific or cure-level treatments for bladder stones in this species. And even with removal, it is difficult to prevent recurrence as you have seen. Generally speaking, management of these cavies tends to be based on pain relief, antibiotic therapy as needed, and modification to the diet (feeding as low calcium/oxalate levels as possible).
Now based on your history, it sounds like your vet and yourself are doing everything you can. Just since you didn't mention your diet approach I would just say that we do tend to feed low calcium and low oxalate diets in these wee ones. Specifically, we want to limit access to alfalfa and instead use timothy-based hays and pellets. Furthermore, we tend to avoid foods like strawberries, spinach, parsley, and celery. As well, we don't want to overfeed Vitamin C (so we need to meet his requirement but don't want to offer excess).
As well, just to note, we do want to make sure that Sarif is taking in a good amount of fluids. The reason is because if we dilute the urine, this will help reduce crystal or sludge build up and potentially slow bladder stone growth. In regards ***** ***** we want him to be taking a minimum of 4.8ml per 100 grams of his body weight daily. So, you can consider measuring his water to see if he is doing so. Otherwise, you can tempt him with Pedialyte or juice flavored water or water rich veggies. That way we can help flush the crystals out and try to reduce the building blocks for his bladder stone.
Otherwise, in regards ***** ***** question about why you are suddenly seeing blood, this is because the stone is catching on the bladder wall. Imagine a jagged or rough edged stone rubbing and bumping along the delicate bladder mucosa. Unlike our skin, the mucosal wall is very delicate and rich in blood vessels. So, everytime that bladder stone is in contact with the bladder wall, it will be eroding and grazing that tissue. And this leads to the bleeds we see. As well, he likely urinates frequently and when handled because its uncomfortable when the stone bumps the wall and he likely has a ladder in a constant state of stimulation/irritation. Its much the same as how any species (cavies or people) have increased urgency and can leak urine with any inflammatory bladder issue.
So, that is likely why we are seeing what we are. And it could be that if the bladder stone is growing to size where its more often bumping the bladder wall or has a really rough edge; then this could be why we are seeing more issues now.
Overall, bladder stones can be a real nightmare for our cavies. And I have to say that your vet is using all the treatments we would for Sarif. From here, the only real options is to see if we can better manage this would be to assess and modify the diet, keep him drinking to dilute his urine and flush out crystals, discuss with his vet whether the Metcam dose can be increased (for better anti-inflammatory effects and pain relief), and even potentially considering additional pain relief if you think he is uncomfortable (despite being a typical prey species and pretending all is well). Because at the end of the day, these wee ones often will have recurrent issues with this and our aim has to be to try and slow stone growth and keep them comfortable despite this unfortunate situation.
Please take care,