If you are a geek at something, it means that you're really into it, so may not have clarity. so if you're a band geek, you're someone who's obsessed with bands, or being in the band, so much so that you lose perspective in this case.
In many schools, the band geeks were not the "popular kids." They were seen as nerdy, and they just played for the athletes, while the athletes were "cool."
This passage says that the father was a nerd, not one of the popular kids, and he regrets that he wasn't more popular then. Therefore he's happy his kids are.
(In my high school, we had a marvelous music program and the kids