In the episode where Stu is making a barbeque, Drew and Grandpa make fun of him. Watch when the scene cuts to Stu, then back to Drew. For about 1 second, Drew has beard exactly like Grandpa then it all of a sudden disappears!
I love these kind of slip-ups. The "glass-is-full-then-it's-gone" are just uninteresting, but when they have a cartoon character growing fifteen eyes and a poop-colored tail out of his shoulder...I vote it a ten!
Mee-moo is right the grandpas don't have beards, unless you count the hair growing out of Louis' ears, and I don't. so maybe Drew grew a beard but it couldn't look like one his dad don't have.