MY Son Who Is 15 Yrs. Has Had A Skin

MY son who is 15 yrs. has had a skin rash which is burning and itch and most of the rash started down on his penis and then spread basically from head to toe. It is not an std, and he has been to 7 doctors who have not diagnosed him whatsoever. One Dr. suggests a staff or bacterial infection but the condition still exists going on to four weeks. Antibiotics did not help. But he has recently been diagnosed with Mono but still no ideas as to what is causing the burning itching rash he still has all over his body. Its not scabies nor any std/ So please , if possible, point me in the right directions.