Suggest Treatment For Itchy Rash

My 22 year old son has had a rash for 2 months +, but he has a life long history of ezcema and we d encouraged him to cut out the foods he had been allergic to as a child. When he showed us that his rash had progressed to itching that was leaving the skin raw on his shoulders and blood on his shirt, we took him to an Urgent Care where the FNP did not take a medical history, or even get within 3 feet of my son. she saw his shoulders and said That is a very bad case of scabies, I can tell you that right now. When we found out the cost of treatment was so expensive, my husband suggested a second opinion at the ER. The ER doctor took a complete medical history, examined the full rash and asked if any others in the house are affected (no, we are not). He also asked about familial history of autoimmunity. I have myasthenia gravis and lupus. My mother has Celiac. His paternal aunt has Rheumatoid arthritis. The ER doctor said (without us knowing we were wanting to confirm the scabies diagnosis) that it was not anything contagious like scabies. He told us that it was psoriasis, which he also has and showed us an identical rash on his arms (not the severity because his is treated and not being scratched raw). He prescribed prednisone. I was so relieved I did not ask 2 important questions. Does he need to be seen by a rheumatologist now, or is it just a matter of keeping the skin condition controlled? And -- on the off chance that the FNP was right, could prednisone make scabies worse? (I don t believe she was, but I m a bit unnerved right now. -- I work in child welfare and it does not look like the rashes I have seen of confirmed scabies in our child clients -- confirmed with skin scrapings).