Thanks for all the other suggestions. I did want to respond to one thing.
Wandering is very kosher and what I am intending.
That simplifies things. He can occasionally pick up a mission when he visits a church, but mostly will be on his own to figure out what Helm wants him to do. If he doesn't care to explore any more in depth than that, that's okay. If he chooses to explore that facet of his character more, the ball's in his court.
Understand where I am coming from this: I am volunteering to run this game at the local teen center (for resume building, and providing young gamers an outlet).
Good on you for that.
These kids have only played D&D a few times before from someone doing the same thing I'm doing. I'm approaching the game as very relaxed in terms of what they do, but certainly have consequences for their actions.
That's good, because I think you're going to need to keep that in mind.
Basically, remember back when you were a kid playing D&D with other kids, and how crazy and carefree it was. I want to facilitate that kind of atmosphere and experience. Perhaps I'm making the mistake of thinking too hard about it.
The only rule I've firmly set up front is "No attacking other PCs."
I certainly do remember. I remember the NEUTRAL GOOD ranger ('cause they were all good, back then) doing something
vilely EVIL to the women of a tribe of Troglodytes. I
still don't want to talk about it (and Eric's Grandmother certainly wouldn't approve!).
My point is this: You might want to set up another boundary up front. Since this is a teen center, whatever behavioral rules that the center has would be a good place to start. Kids, if left to their own devices, are going to search for boundaries and push against them. The kid playing the evil character has already demonstrated that the boundary he will be pushing is a moral one. If there
is no moral boundary in the game (as "no attacking other PCs" is a [wise]
practical boundary), that player is likely to keep going and going.
Furthermore, this amounts to an indirect form of attacking the other PCs, as it forces those PCs to face the direct consequences of his actions (when there are some--the player may become adept at making sure no one in-game finds out) and otherwise forces them to play a tone of game they may not wish to play. (I certainly didn't--even as a kid.)
I don't think this player is really deeply invested and Helm matters all that much. I even said "hey come up with your own deity". He just sort of shrugged. So to him I don't think it's all that important, but I want to try and inspire.
Your interest is important and valid, too. Bored DMs run boring games, after all!