Mario Golf is a good game too, though I haven't played it in a while. I'm trying to imagine that game with the Wii Sports golf controls. Mmmmm, goodness... You know it is coming at some point, anyway (Wii Mario Golf that is).
I second the Rogue Squadron games - still pretty much my favorite GC games (except the on foot missions in the second one). I loved Fire Emblem, Paper Mario. I just started playing Batallion Wars - it can be fairly challenging. I thought Star Fox Assault was excellent - don't let all the haters get to you. If your son liked the demo, the game is pretty much like that - yes, there are some on foot portions that aren't as good, but they aren't that bad either (like the ones in the Star Wars game). I might not have recommended it at $50, but at $20 or less it is definitely worth it.
Let's see, some others: Pikmin and Pikmin 2, Super Mario Sunshine (I still like this WAY better than Mario 64, which I'm playing on VC right now - most people prefer 64 because they played it as a kid or something. Since I missed it and played Sunshine first I think Mario 64 is a pale imitation
).
Meh, there's plenty of others, but you've got some good ones to start with.
Before I saw this thread I was wondering what Wii sales would do for GC software sales. Looks like, in this case, it is helping. Chalk up another genius point for Nintendo.