It sounds like an allergic reaction. I would take him to the vet and check his collar.
My pit had this also but i started bringing him inside more than he is outside. and changed his food to blue buffalo. the cheaper food was not good for him. hes doing so good now.
Allergies can be VERY frustrating to diagnose. It could be his food. Most foods have a lot of grains in them and a lot of poultry or poultry by-products. These things are responsible for a large number of allergies. The easiest way to see if it's a food allergy is to switch him to something he has never eaten before, like venison or a food based in fish and potato. Look for one that doesn't contain corn or grains. If it's a food allergy, once you remove the allergen from his diet, you should see an improvement very quickly.
Does he have a crate mat in his cage that you wash? If so, it could be the detergent that you use or it could be a freshener that you're using, like Febreeze or one of those. Try eliminating that if you are using it to see if you see a difference.
You can also take him to the vet to see what he thinks it could be. Please keep us posted as to how he does.