My Son Has A Horrible Rash Covering Nearly His Entire

My son has a horrible rash covering nearly his entire body and is accompanied by redness of skin and very swollen feet and ankles. We live in a rural area where the healthcare leaves a lot to be desired and was told by a physician at the local ER that it was poison sumac at which time he administered 2 steroid injections but gave him nothing to follow up such as prednisone. This occurred approximately 5 weeks ago and since then the rash has worsened. Any ideas?