Could be an allergic recreation but there's no way to be sure. Does your brother usually take hydrocodone / does your brother usually take that much hydrocodone when he does take hydrocodone?
Rashes can be a sign of an allergic reaction, for sure.
Just speculating here (I know, bad!) but if you're taking 60mg+ hydrocodone what dosage form is the hydrocodone in? Is your brother just taking twelve 5/500 Vicodins or six 10/325 Norcos or is he using the Cold Water Extraction method?
If he's just popping the pills with a large amount of acetaminophen (APAP, or Tylenol) that could be causing the reaction or at least making it worse.
..I hope he's not taking twelve 5/500 Vicodins, that would put him at 6000mg APAP, which is way above the maximum daily recommended intake of 4000mg.