My bf’s boss texted him at 10pm to come in at 4am for a job tomorrow.

Basically what the title says. He works “on-call” but doesn’t get paid as an on-call employee. He works in the oil field. His boss texted him tonight at 10pm letting him know about a job he needs to be at tomorrow at 4am. Is this legal?? I CANNOT find the laws regarding this. We are in Ohio. His employer is the worst. Beyond messed up.