Unless he’s a salary employee. In that case, he’s technically always on the clock or could be argued to be so. If you want your time to yourself then stay an hourly employee.
IANAL, but it's definitely not that simple. I've worked in salaried positions and still retained ownership of my side projects. There was also one instance where I refused to sign a NCA that threatened my IP, so YMMV.
A salaried position is usually for X hours per month though. An employer that can make you work 24/7 each day of the month is not an employer, they're an owner.