He can't sit still. He's restless. SOURCE: Bernstein, Jüdische Sprichwörter und Redensarten. Compare:
He's sitting on pins. | Er zitst af shpilkes. | .ער זיצט אויף שפּילקעס
SOURCE: Stutchkoff, Der Oytser fun der Yidisher Shprakh
ער האָט שפּילקעס אין תּחת.
Er hot shpilkes in tokhes.
He's got pins in his behind.