I'm at the stage where the next course of action would be to have all his teeth removed - a major, major couple of operations on a 14 yr old cat with no guarantee it would cure it.
The vet said something about how in some cases it comes to that. I couldn't believe it! I've had cats all my life and haven't gone through this.
I did recently change their litter from the wheatscoop to this cheaper sawdust stuff at the grocery store. I'm vacuuming around the house up all the little dusties and going to change it back to wheatscoop to see if that might be it.