Watch him closely, if he has crystals he has progressed from a plain UTI to FUS - Feline Urological Syndrome. This will be a lifelong condition, but very often a change of diet alone will halt all symptoms. This is the condition that can cause the urethra to block, which can become fatal in less than a day. The minute you notice him straining, or if you ever notice he has a hard little ball-like lump way down on his abdomen (meaning he can't empty his bladder), take him to the vet immediately. He's in our prayers, but I'm sure he'll be okay with the good loving care you are giving him.