Doctors don't know what is causing rash

Husband, age 55, 7 days with fever 102.5, extreme fatigue, loss of appetite. NO symptoms of congestion, pain, swollen lymph nodes, diarrhea, or vomiting. Fever broke and feeling much better (not normal yet) but now he is covered with rash which looks like hives but are NOT raised, itchy or painful. He wouldn't know they were there if he couldn't see them. Went to ER; tested blood and urine. Normal blood count. Negative for strep and mono. Very small amount of blood in urine. Normal chest xray. ER doctor assumes infection but did not treat and referred to PCP. PCP reviewed tests and called in another doctor. Neither know what is causing the symptoms. Referred to dermatologist; seeing that doctor tomorrow. The only meds he takes is metroprolol, for hypertension, and he has been taking this for years. Recently exposed to sooty dirt from 160 year old house (replacing windows) and sanding paint from same house. Any ideas?