One of our cats, Oreo, has very bad teeth. He's already had plenty of them pulled, and still has sore gums. The vets say that he may also have bone loss and tissue problems, and the only treatment is for life and costs 120 dollars a month. And it's probably too late for the treatment to be effective in any way--he may have to have all his back teeth on both jaws pulled out.
The most likely cause: his dry cat food. Don't buy Science Diet cat food. Already we know of one person whose cat had skin problems that went away after they stopped eating it. Plus, they own a rendering plant that turns euthanized, sick cats and dogs into cat food! Evidently they haven't learned from the BSE scare.
In short, don't buy Science Diet!