A new cry

My industrious little baby has found a new reason to cry this week. He's taken to crying because he can't crawl. Patience is not currently his strong suit.

He's been happily playing in his crib. Three times now I've left him in there wiggling on his back and playing with his feet and come back two minutes later to find him on the other side of the crib, sort of up on all fours and pissed because he can't go forward. He looks up at me and then drops his head and cries some more. I really don't know what to do in this situation. I know he knows how to flip himself back onto his back, and he's not going to learn how to crawl if I pick him up with these cries. He just gets so sad and frustrated. I've taken to cheering him on, but I don't think he understands the encouragement.