Your dog is biting its tail raw? Yes, we had a collie like this. We used bandages wrapped around its tail and secured in place with duct tape.
It was the only think we could do to stop it biting its tail raw.
You are also recommended to bathe its tail in warm salt water (kills bacteria), and you may want to try some flea-treatment medication just in case that is the problem.
The "bandage and wash" method is something we had to repeat for several months. Eventually, we tried removing it and she was fine with it.
I was very happy because the vet said the alternative would be to get its tail docked :( I wasn't to keen on that idea as you can imagine!