Maybe his reply settings and actual email are different? That happend to me just the other day when I set up a new computer at work. People could send to me direct, but not reply to a message.
Etek 3LT grey/black f/s
Valken Proton greyish/black f/s
68 4500 PE stubby, LP output f/s
68 4500 AIR tank(no reg) f/s
Creed Battery, eyes and noid f/s