I need some specific advice regarding diet for my 12 year old cat.

I recently created this thread, and missye87's response plus my own further research has led me to the conclusion that he is probably suffering from intermittent stomatitis. (I'm not going to drag him to the vet right now for tests because his mouth is currently fine, and he hates vet visits.)

On the page that missye87 links to, there are 3 foods recommended that can help with this. Of the 3, the only one that I can get, and Sam will eat, is Applaws.

I've always fed my cats a variety of wet foods, both to keep their interest in food, and to hopefully provide them with all the nutrients they need. My concern here is that Applaws is listed on the wet food A to Z as a complementary food, not a complete diet.

I'd like to try regulating his diet in the hope that it will help, but I'm worried that:

1 - restricting him to one food will deprive him of essential nutrients

2 - I'm also not sure that he will tolerate just Applaws. He gets bored easily, and likes strong smelling, meaty food with gravy best

3 - I also like the wetness of food with gravy, especially in the summer here when it's very hot, because I know he's getting plenty of fluids - although I'm not sure how much fluid loss in high temperatures is an issue in cats

I'm just wondering what my options are really. I'd like to control this as much as I can without dragging him to the vets every couple of weeks.