He may have allergies, fleas, a bug bite, sore or it could be a habit, best to get him to the vet to be diagnosed and treated properly. Best wishes.
Why Does My Dog Keep Licking His Tail?
Yes go to the vet it sounds like allergies to me.my dog chewed his tail down to the skin and then some he did it in like a day,don't wait go and he will be much happier.