A sheltered teenage nerd, convinced he’s the son of a missing heavy metal legend, joins a band of misfit headbangers to carry on his father’s legacy.

Since you make a point of saying "he's convinced", I'm going to assume he isn't the son of the legend, but the result is he blossoms and becomes a star in his own right. A coccoon-to-butterfly transformation. If that's the case, I think it works as an effective logline. It's under thirty words, and hits most of the elements. I think the concept might be original enough and certainly low-production enough, to draw interest. You might improve it slightly by adding "when" in the beginning. This forces you to include an obstacle and ratchet up the conflict.

Good job!

Geno Scala (sharkeatingman), judge.

I think the logline might need to reveal whether he actually is the son of the aforesaid legend as I immediately wonder about this (which is good), but the logline doesn't tell me what it is. I think it could do with a more cogent and obvious goal too as carrying on a legacy is pretty vague. Maybe his "father" has a reputation he needs to restore? I assume (but your logline could make more pointed) that finding the missing father is also a goal?

I like it. I see some good comedic potential.