He may need his anal glands to be expressed. If he begins to suddenly notive his rear and try to chew beneath his tail, this is a sign. Absessed anal glands are a common problem with toy breeds, and Chewy has had to have them expressed once. It's so common that many groomers offer this service. Here's some material on the subject...
http://www.barkbytes.com/medical/med0029.htm
Enjoy the delightful illustration, lol...