Damage to legs from Henoch-Schonlein purpura? Stumped the doctors

My son, 13 at the time, was diagnosed with HSP vasculitis February of 2018. Took several days to diagnose because he did not present with purpura rash on legs. He was in the hospital for 14 days. After 9 days of uncontrolled pain (including nerve blocks, morphine and hydrocodone) he was transferred to Vanderbilt in Nashville and was treated with Toradol, released 3 days later. Then treated with steroids for 4 months. 60 mg for a month, then 40, then 20, then 10. He seemed to recover. We then moved to Chicago in June 2018. July of 2018 he would complain when standing that his legs itched. Went to see Rheumatologist who said if it got worse to a vein dr. Legs only seemed to itch when standing still so my son did not want to see a dr. August 2019 the itching of the lower legs has started to get worse when standing. His legs turn bright red, but subside when he moves or sits down with legs up. By January 2020 his legs have become extremely painful when standing still for just a few minutes. PC doctor tried to get us into a Vein Doctor, but no one would see him because he was "pediatric". He is now 15 and over 6 feet tall. I continued to call around to all kinds of doctors trying to find SOMEONE to see him as his symptoms got worse. Finally got an Interventional radiologist at Lurie Children's Hospital to see him. He was able to watch my son's legs turn bright red between his ankles and knees as he stood still for the ultra sound. He said the ultrasound appeared normal, except for slightly narrow veins, and that he had never seen anything exactly like my son's issues. He referred us to a Vein doctor at Northwestern for more tests speculating the HSP may have done damage to the valves in the veins. The doctor at Northwestern did a more detailed ultrasound of the leg "simulating" standing by using cuffs around his ankles. The test did not produce the pain or reaction that normally happens in his legs while standing. The vein doctor also said he had also not seen anything quite like this, but the valves seem to be working normally. He asked my son if compression socks helped. My son said "no" and the doctor said that further confirmed that it most likely was not a valve issue. We were referred us back to a rheumatologist at Lurie Children's Hospital. His symptoms have steadily gotten worse. Started out as just itching, then a little bit red, then purple legs between ankles and knees. Now it has travelled into his feet and just above his knees and has been more and more painful. His legs turn crazy colors or red and purple and the pain has increased from itching to tingling to very painful now. The pain will subside if he gets up and moves around or sits down with his feet up in a recliner. It used to only happen when he stood still, now it happens while he is stilling down playing Tuba. He also complains of lack of breath when plays now. He sometimes gets really bad leg cramps. We are still waiting for an appointment because Covid19 has now happened we do not know when we will actually get him in. I have several pictures of his legs and didn't know if there was a way to post those. I have read all the different vasculitis and none of them seem to fit. It is getting to a point where it bothers him multiple times daily now. He is getting angry and frustrated as it gets worse. As a mother I worry about long term issues and know how much pain he was in the first time and do NOT every want to see him there again. Any ideas from anyone? He loves marching band and really wants to try out for DCI, but that requires that he be able to stand still and also be extremely active. At this time he can do neither.