Demodectic mange does seem to be quite pervasive among badly bred pit bulls, if you're sure this is what he has. The mange mite lives deep in the skin of animals and doesn't usually cause a problem unless the animal is in poor heath from malnourishment etc or the animal is very young.
This is basically a disease of puppyhood, and the problem usually resolves itself as the dog gets older and it's immune system becomes mature - this happens between 12 and 18 months normally.
A raw diet with added yogurt can be helpful.
When you take him to be neutered, ask the vet if the anesthetic will further supress his immune system, worsening the problem. It might be better to wait til he is over this. Many drugs and chemicals used also exacerbate the problem.
Go to Google and type in "Demodectic Mange natural remedies" without the quotation marks and you'll find lots of good info.
Just so you don't give up hope, here is Buddy, a dog who was dumped with severe demodectic mange. These are his "Before" and "after" pics.