If he's chewing on his bottom he might need to have his anal glands squeezed. You should talk to your vet.
Your dog could be suffering from food allergies. I also had this issue with one of my dogs. Best remedy was to go to a raw diet. No bum licking and a whole host of other benefits, i.e. better breath, better teeth, less shedding, flea resistent (after 6 months on the diet). Look into it. One of the best suggestions I ever had and I will never go back to commercial dog food again.
Best remedy "raw diet". What exactly do you mean? I feed my boxer Purina Lamb & Rice. She likes it. I just wonder if that may be causing her to itch constantly.