Professional Ordnance carbon fiber AR-15's were junk and, due to a bunch of proprietary parts, they were incompatible with most standard AR-15 parts/accessories. When Bushmaster took them over years ago they went through the gun completely and changed everything to standard (mil-spec) dimensions and also made them reliable. The Bushmaster guns are good shooters, I used a C15-M4 as my patrol rifle for several years.
Your brother wants too much money (especially since you're his brother) for what he has. Bud's Gun Shop has brand new flat top Carbon 15s with a red dot for $699 right now.