Cat mouth ulcers

Ok- so there are a few cat lovers who pop in here from time to time. My little guy Pablo has a mouth problem where he gets ulcers on edge of his mouth. He was good for a long time and now has one again, so I decided to do a little internet research into what causes this. I don't have a solution yet. I am trying a new food along with their old food because they (him and his sister Lucy) are almost 2 now and I would be bored after eating the same basic food for 2 years. This could be the cause.

The best site that I found was from a doctor who invited people to comment. He has an email from a nurse to which his response is very funny to me (jerk) : http://www.2ndchance.info/ulcers.htm