bruises or sores on cat's lips

My 7 month old kitten has an issue with his lips. A few weeks ago, his upper lip was swollen, red and looked like a sore. The vet gave him an antibiotic and it cleared up within a week. Now, his lower lip seems to be doing the same thing. It's pink/purple, kind of like a bruise would look as it healed, ie., discolored. the fur on his chin is a little thin there, too.

I keep reading about rodent ulcer. My vet mentioned it as a possibility when I took him in last time. How do you know for sure that's what it is? Can it be cured completely? If so, how?

What else could it be?

I am taking him back into the vet on Monday, but how can I be sure that this gets diagnosed properly?

Thanks in advance

PS... a few things that may or may not be related: He is a long-haired cat and is constantly grooming and 'licking' things: himself, our other cat and me. When I'm asleep he licks my face and earlobes.

It could be an issue with the food bowl he eats or drinks out of.
I remember reading an article about how some food bowl material causes sores in cats.
Unfortunately, I forgot where I read this.
Maybe you can ask your vet about it?

Thanks for the replies.

I've always used stainless steel, although they do drink out of one of those Drinkwell fountains which is plastic. I've also started cleaning and changing out the stainless bowls every day, rather than every few days.

I'm definitely going to talk to the vet about the possible causes. I just thought I could go in with some concrete information so that the vet doesn't just prescribe an antibiotic when the diagnosis should be something different.