Info on cat with (Possible) Urinary Tract Infection?

My 3 year old cat has been urinating a lot lately, and my mom believes he has Urinary Tract Infection. I've been reading that early symptoms include straining and frequent visits to the litter box. I know I should make sure he has clean water and a clean litter box, but what about his diet? My 2 cats (one female who is healthy, one male who may have the infection) currently eat Friskies, but is there another brand I should buy? And are there other symptoms I should look for? Do the symptoms differ for how long the cat has had Urinary Tract Infection? My cat will be taken to vet soon, but your input would be greatly appreciated nonetheless!