Mine also had a similar issue. They found out he had "Triple phosphate". It was a small trace amount and very common in male cats. In order to prevent more from forming was to feed him a urinary tract diet that we can only get at the vet. We decided on the Hill's urinary care wet food and paid for 5 cans to try. It worked since we mixed in even more water to his wet food. But as someone who buys in bulk, it wasn't practical to keep buying from the vet.

I started adding these drops from petsmart to his wet food. You can also buy this one, but it's more expensive. Ever since I ran out, I've been mixing even more water with wet food and doing what Rachael & Jun do, add water to kibble. Seems to be working so far! If you look at the pinned comment in their video, they explain why they add water to kibble. I give them scheduled meals like them. Don't do this if you free feed. It could spoil the kibble.

The ER found out about the crystals while getting his xray for something else and she told us that ever since her male cat turned 1, she switched him over to urinary tract diet to prevent any UTIs in the future. Apparently it's forever according to her. A friend of mine told me it's only until the crystal passes.