Instead of just telling him what his faults are, let him know what you guys are looking for in a game. He should be able to pick up on that and say to himself, "Yeah, I need to do that more often". If his DM'ing is ok but you just don't like the campaign, just ask him if he'd run something more geared to what everyone else wants. It's possible he wouldn't want to do that and if so, either deal with it, DM yourself, or find another game. Any good friend should understand that not everyone will like to play in his game.
I had players try to get me to run a different campaign and that just won't happen. I love Planescape and I spend too much time learning that campaign to just throw it all away and spend time learning a new campaign. So they quit my game, which is perfectly ok with me. I don't expect everyone to like my campaign.