When they bite over the hind quarters, and hips, it is usually due to flea saliva allergy.
If he is biting around his rectal area, that is usually food allergy.
When dogs are allergic to flea saliva, one bite cause them to have a big allergic reaction. Your vet could test him to see if he is allergic to flea saliva. If the hair loss is over the hips, then I would assume it is flea saliva allergy until proven otherwise. Make sure you put the advantix on every 4 weeks. If you bathe him, you will have to apply the advantix more frequently. If the advantix is not controlling the fleas, you can add a product like "comfortis" or "capstar." Both are oral medications. You will have to get the comfortis from a vet with a prescription. You can get the capstar, online, without a prescription.
You could try putting him on a hypo-allergenic diet. This is different than a natural dog food. The most common food allergens are beef, chicken, lamb, corn, and wheat. The name of the hypo-allergenic dog food is "hills z/d." You will have to get it from the vet. He will be on the hill's z/d for 6 weeks. If his skin improves, you know he has food allergies.
Dog's skin often turns black with chronic inflammation. It will return to the normal color when the itching and inflammation resolve.
To help with the itching, you can give 50 mg of benadryl every 8-12 hours.
You can spray hydrocortisone spray, on the affected areas, every few hours until the skin is no longer itchy or inflamed.
I hope this helps.