Thanks for all the advice so far guys. Good stuff, really helpful. I really like what Mesh Kong said about the Dm giving him more direction, because our DM IS extra open ended, so that should help with NPC interaction.
To answer a few questions: I am NOT the DM, but I am the most experienced player. My character is not the leader, but I guess you could say I myself am kind of the "party manager". That's why I'm trying to get this worked out.
Sadly the player in question is NOT 9 or 12. We're all in our 20s.
He plays a Priest Gunslinger who hunts undead and supernatural monsters. (Paladin/Fighter, and our DM even gave him favored enemy Undead for free.) He doesn't QUITE seem to have his background and personality ironed out.
Anyway, like I said before everyone (including the complaining player) are having a blast with this game, it is seriously awesome. His whining was annoying, but not unbearable, until last session when he lost 2 potions in a fire and was so pissy we thought he was going to quit, really ruined it for everyone.
The crazy thing is, he really is a cool guy, and a good, fun player too for the most part.
I've given him a day to cool off and I plan on calling him this weekend and working everything out. I'd already mentioned his complaining and it seemed to cause a bigger disaster, so hopefully we can work this out.
Thanks for the advice, and I'll let you know how it goes.