It sounds like your cat may have peridontitis.
In the primary stage of peridontitis, the gums (in your cat's case his lower gums) are inflamed and swollen and appear like a "fat lip". Treatment by a veterinary dentist can reverse your cat's condition. Check either on the web or in your local telephone directory for a member of the American Veterinary Dental Society. These are veterinarians with a specialty in animal dentistry.
In the meantime, I recommend Petrodex Enzymatic Cat Toothpaste.
Veterinarian
D.V.M. for more than 30 years. I have experience treating all varieties of animals