What Causes Skin Lesions With Joint Pains?

Question: Well I need someone to talk me off the ledge. My 21 year old son away at college developed mild to moderate joint pain (it acually started in his foot), then traveled to knee, ankle and sometimes fingers. Went to school clinic. She noticed a small rash cluster on his back and thought it was Lyme. Treated for one week Meanwhile rash grew until covered entire chest, large pink oval or circular shapes and back too. Nothing on face or neck or leg. Did not itch.. He followed up at an infectious disease specialist, who tested for Lyme, among many other things and of course general blood tests. Everything came back normal/ except one marker indicating significant inflammation. Doctor thought probably a viral infection that would run it's course but referred him to dermatologist to look at rash. His apt. there is in one week. He had about a week of complete disappearance of rash and joint pain. Then rash came back with the joint pain-always together. Went back to infectious disease guy and he took more blood, retested for Lyme. He also thought rash resembled ptyriarosea (sp) so tested for herpes as well. Again, everything came back normal, except still evidence of inflammation (joints I assume) The herpes test will not be back till Friday so do not know about that. I should say that otherwise he really is not sick. His joint pain is almost completely controlled by Ibuprofon and he is going to classes, walking a lot, texting al the time so he is fully functional and feels otherwise okay He says the rash tends to worsen then recede. When it recedes it appearance changes and almost looks like hives. so what are your thoughts? Could it be an autoimmune thing. My first thought was rheumatoid arthritis but please know his joints are not red, hot or swollen. Maybe another autoimmune disease. Lupus does not fit, no butterfly rash, facial rash or other symptoms. He is just focused on his studies I am the worried mom. Any help is welcome!!