In Feb-2000 I was in a nasty snowmobile accident up in the UP of Michigan and flew back on Mesaba+NW (CMX->DTW->BWI). I had metal pins sticking out of an arm cast and my facial bones were "floating" (think cracked egg shells-- I was on my way to a 13hr facial operation). Folks were concerned about the effects of the low pressure on my bones. (shattered humerus and panfacial fracture)
I flew as a normal PAX in both coach (in the Saab) and first (DC-9?) and I'll tellya-- it made no difference to me. Sitting uncomfortably didn't even hit the pain radar screen. The last thing I wanted was to drink or eat much either.
What made all the difference in the world was the plentiful supply of pain meds I brought with me. I popped one right after security (ahh, pre-9/11 security when a cast and pins elicited "you're good to go, sir") and enjoyed... well, I don't quite recall what I enjoyed... but I wasn't bothered by flying coach.
Your brother will be okay. His MD should find it reasonable to allow him some "emergency pain medication" for the flight. ( :-: Just be sure he tries one before actually flying... 37,000 feet is a horrible place to learn that your body doesn't react well to a medication. :-: )