Did he always not wag his tail or has this started recently? I just ask because we've had 3 dogs and our first, who was a big tail wagger (all you had to do was start to say "walk" and it would start thumping on the floor) one day woke up and his tail didn't wag at all. It just hung there. It was so odd we called the vet and were told that sometimes dogs pinch a nerve in the tail and it effectively paralyzes their tail for a while. With Bigsby the wagging slowly came back and after about a month it was normal again. It happened one other time in his life after that.
Our second dog was less of a wager than our first, but it still wagged. Our current 4 month old pup wags her tail when she is happy and excited but I can't yet get her to wag it just by talking to her in a silly voice. Her tail is very expressive: I can tell if she's super happy, shy happy, a little nervous, sad because she just realized we are leaving, etc..