Ideally someone who never broke huge but was in 100 episodes of something.

Jimmi Simpson might be good.

I could wish for DJ Qualls but despite his age no one would ever believe he has an adult son.

Jason Bateman?

No, I don't like his face, but right direction.

Character is a nerdy, self-defeating pessimist with no real skills who somehow wound up integral to repeated savings-of-the-world.

Early-mid 40s, worked in a DMV, alienated his son with his anger issues.

Zach Braff

Strong contendor