@jsimmons74 - It is very helpful to see the different photos. Your son must really suffer- the photos of the below knee angry red ,raised inflamed skin were almost shocking in the degree of inflammation. It does look like vasculitis, especially also with pain. I assume this is a continuation of the HSP vasculitis, but different. There is another painful nodular inflammation of the the lower extremities called erythema nodosum.
It seems as if he would need to have a skin biopsy to see what actually is going on under the skin surface. I hope with a diagnosis he can be treated with anti- inflammatory or anti- immune medication. He did respond to prednisone. I have just written down my thoughts here -I have dealt with different autoimmune conditions. Hope you can get to see someone soon. The photos are very helpful. Please stay in touch!