Thought of a few more:
He would argue with a fence post.
Dumber than dirt.
Good Grief
If a frog had wings, he could get bird booty.
Scarce as hens teeth.
If I tell you a chicken dips snuff, you better look under her wing.
Blinks like a toad in a hail storm
Panting like a road lizzard.
If you're not out front-the scenery never changes.