My 10 Year Old Son Has Myositis And Has Acute

My 10 year old son has myositis and has acute pain in his upper thighs. He can't put any wight on his legs and also complains of headaches and stomach pain. He's had these symptoms for 6 days and I'm wondering when it will clear up. We went to ER and were told it should take 5 days. He's showing to progress. Thank you.
Anita