Colt 6920. You get the quality and reputation of Colt, and they are competing actively in the market. $750 +/- gets you a full AR with Mike-spec everything.
for less, you can get other great weapons. Rock River was mentioned. Stag is another good choice. PSA assembles a great firearm. Since this is his first, I would recommend Colt since he will know the name, and he will never have to ask about whether something will fit in his gun, or replacement or upgrade parts.