Hmmmm....our guy chases his tail once in awhile, then gnaws/locks furiously on it when he catches it (as if he's got some great itch). But the funny thing is, our breeder says his dad does the same thing...I was going to mention it to our vet at our next visit.
I'd at least call your vet and ask, especially since the fur is disappearing...could have a mite...does he scooch his bum on the floor? Could be an anal gland thing...though, not sure which part of tail he's munching...