Bear in mind, it's not your job to "grow your group" If you sit down every week with the same 5 players, that's your group. What other groups exist (whether the same people or not) is not your direct problem.
It's not your job to make sure he runs a good game.
Nor is it your job to tell everyone he's invited that he sux.
If he doesn't ask for your help, you can't give it. You can try, but if he really wanted it, he'd be following it (and not running a game yet). So he's not listening
Obviously, you care about increasing the number of D&D players as a whole. You also care about giving new people a good impression. So what can you really do, that will have an impact?
If he lets you play in his game (if you tick him off with pesterings, he might not want you), you can:
be a good player and follow his plot hooks
don't quote rules unless he asks you for them
don't argue the rules with him during the game (you'll give the new people a bad taste)
After the game:
if he asks for an opinion, suggest what might make it better (avoid saying "this was bad")
if the game still totally bombed, talk to the new people privately and suggest that they try different GMs until they find one they are comfortable with.
Basically its damage control. You don't make his flaws obvious during the game. You don't talk about his flaws directly, you indicate how it can be made better.
I have taken over a new group when the designated DM sucked and I saw the players who knew it sucked. I talked to them afterwards and started up a campaign with them and they dropped out of the other guy. We did it under the guise of splitting the workload up, so more people can play. In your case, just steer them to another DM, if this one doesn't work out.
One last thing to remember, the first DMs had no experienced players or experienced DMs to emulate. The first DMs had first players and none of them knew what it was about. The smart ones figured out their mistakes and improved for the second game. The dumb ones eventually run out of players.
Janx