Well he could have periodontal disease. Have you brushed his teeth all his life? Is he on dry dog food? Do you get dental cleanings regularly? If you have said no to any of these questions then well he could very well have a dental problem. Without seeing your dogs teeth I cannot tell you the exact reason. I have a pom myself. I get her teeth cleaned every few years (kind of expensive but I can't stand the fish smell). Her teeth aren't in the greatest shape and I know it's because I didn't do what I was taught to do in school (brush teeth, feed dry food, and all that jazz). There may be nothing you could do about it now that it has advanced to the stage of tooth loss but I would definitely take him to a vet to get a full checkup. It may be due to an underlying issue.
