my cat has allergic cystitis

my vet says my cat Bailey has allergic cystitis. This comes after bladder stones, crystals in his urine despite prescription food, and a penile urothstomy. I am thinking after some research, that he may have a problem with grains and would like to put him on an all wet diet that has no grain in it to see if that works. He pees everywhere whenever his bladder becomes inflamed. (no bacteria in his urine, by the way) HELP!!! What kind of food do I choose.

Now you have me wondering if that is Cocos Prob because her test is Neg now and she still has blood. Its been months.

My cat Bailey first got bladder stones at 1 year old,(had surgery to remove them) then they put him on prescription food to lower his ph and told me that it should fix it. Exactly one year later he started having difficulty urinating so they put him on steroids because he didn't have an infection. That worked for a while. About 4 months later he stopped peeing altogether. They told me that his urethra was scarred due to the crystals he still had in his urine, even though the prescription food had lowered his ph. Our only choices were remove his penis or put him down. He is such a sweet guy, of course we did the surgery. That was this last November.
Just this week we got another bout of cystitis (no infection) and the vet told me that his bladder was thick and she felt that it was allergic cysitis. After looking online, I discovered that some animals have allergies to grains and maybe this was our problem. I had him on Wellness when I first got him and that has very little grain in it, but maybe I have to cut it out altogether. I am at my wits end. I love him, but I just can't have him peeing all over my house. (my walls, rugs, evens counters. GROSS!) It is too big a project to do the pee search in the dark with the black light on a constant basis. I would really like to fix this for him and me.