What can my father do?

My father is working full-time for a fabrication company. However, with the recent economic cirstances, his company just layed a bunch of people off. My dad gets paid salary, so he isn't paid by the hour... But after the lay-offs, the company makes him work extra hours every day to cover for the people who were laid off. He works overtime, but doesn't compensated. Surely that can't be legal... What can he do?