If he isn't on flea preventative, then he might have fleas. First thing is to definitely rule out fleas. If your dog is on advantage or frontline, you can skip this step. If they are not, you should at least explore the possibility of fleas by examining your dog's skin especially around the rump area under a bright light. Fleas are fast and are a reddish brown color so you have to look closely. Frequently you will only see flea dirt (feces). If you find fleas dirt or what you think is flea dirt, get a moist white paper towel and rub it on the area. If it turns reddish color then it most likely your dog has fleas. A flea comb or lice comb run through the area may capture a live flea as well.
However, pitbulls are one of the breeds that are more prone to developing allergies and demodectic mange. Both of these conditions can lead to itching and even hair loss if it continues for too long.
There are two types of mange and mange is actually caused by mites. Usually hair loss starts in specific areas of the body before spreading to other areas. Allergies are a frequent cause of scratching and hair loss and an be caused by many different things. It could be from a change in diet, or even a shampoo, or other substance. It is hard to pin down what a dog is allergic unless you do testing.
Here is two websites on Mange.
You can read up on allergies here:
If you suspect an allergic reaction you can give benadryl at a dosage of 1mg per pound every 8 hours which can be repeated once if it doesn't have an affect. If you decide to switch food, it may take several weeks to see improvement after the change of food..
There might be a secondary skin condition in addition to the allergy such as staph dermatitis or yeast dermatitis. Staph usually occurs on the lower regions of your pet and tends to have small pimple type bumps. Shampoo containing Chlorhexiderm and/or Oatmeal can help with this condition though it does not cure the allergy. Yeast typically shows as a greasy area that has a sweet musty odor. Sometimes the skin can become inflamed, darker and thickened due to itching. Yeast likes areas such as between toes, armpits and ears. Selsun Blue Shampoo can help with Yeast dermatitis. When shampooing, lather and leave on 15 minutes before rinsing. These shampoos are not meant to be a cure, just a relief until your pet can see a Vet.
If the benadryl seems to help, then it is likely an allergic reaction to something. If there is no improvement, you may want to have your vet do a skin scraping to check for mange. I hope this information is helpful to you.