Is the hairloss and irritation at the tail base on the back? If so, I would make sure you don't have ANY fleas. That area is VERY common to have hair loss and irritation with a flea problem. I know they can be hard to find, but if you see little black specks on your dog's skin if you move the hair back, you have fleas. Those tiny black specks are flea feces. Do you have him on a good flea preventative?
If this is not the case, it could be an allergy to something or low thyroid. Allergies are difficult to determine and deal with, but it's easy to check for fleas and easy to check for thyroid problems, so I'd try those two options out first. If no problems with those, you may have yourself an allergic mastiff... if that winds up being the case, I can give you some pointers on that as I have a VERY allergic APBT.
Keep us posted on what you find out and good luck!