My dog is biting and licking his backside uncontrollably?

Hes biting his bum and back legs almost 24 hours a day.Its been going on like this for about a year. ive taken him to the vet around 10-15 times in the past year to get his glands emptied, it was less than 3 weeks ago when he last had them done. He is running round with his bum curved to the side ready to bite and wiping himself all over the furniture, he's bitten it down so much its started to bleed. I'm fed up with going to the vets and them telling me its the same thing, i feed him lots of fibre bran so that his poo is harder and helps to empty them naturally. I'm at my wits end with the biting and darting round..hes clearly in alot of discomfort but the vet cant seem to help ive tried 2 different vets and they say the same thing. He is no different after hes had them emptied its just constant and it must be driving him mad but i dont know what else to do! Hes definitely not go worms or fleas since hes treated for that regularly.