If you're going to have a wisecriacking older mentor...
Make sure you give him funny material. Sam Rockwell was an exact copy of Bill Murray in Meatballs and countless other older-guy-befriends-the-teen. Sam's lines were silly, but not funny.
When he stood up to Steve Carrell and said "I'm the friend of the 3," that was his only great line of the movie. He needed more.