Been there before OP, cat had to have surgery to remove uroliths as they were blocking his urethra, they're like grains of sand. he was put on urinary SO food, and after 3 years he had an episode of UTI and xray showed the damn crystals again, switched him to Hills CD stress food and it dissolved the crystals, so try him on the CD stress food (chewy.com) and hopefully you won't have to get surgery, and don't feed anything but the CD stress as ant other food or treats can cause crystals.

Good luck