The doctors are stumped. Blood tests are clear, what in the world could be causing this rash?

We came back from a medical mission to Guatemala. My husband first got a terrible sinus cold, them got this horrible rash. It started on his flanks. It doesn’t blister. It just itches so bad that he will have bruised streaks in the morning from scratching in his sleep.
It seems to be symmetrical. If it is on his right flank it will be in his left flank. It seems to stay there for a while them migrate to his thighs, calves, back, arms, always on both sides of his body in the same place.
They put him on prednisone and it knocked it down, but as soon as he started to taper down it came back. Prednisone is nasty stuff and I know he doesn’t want to stay on that for long.
I realize that you are not doctors, I am just trying to brainstorm to see if there is something someone might have heard of that the doctors are missing.
Ruled out valley fever (we live in an area where it’s prevalent) and scabies. Did a crap load of blood tests and a chest x-ray.
Where’s Dr. House when you need him??