California is at will employment. They can fire him for parting his hair the wrong way if they want.

I'm curious, but surely there are some rules regarding, for example, the number of hours per week of work, right?

Employers can't just ask anything from their employees, like being available around the clock and that kind of thing I suppose. Or can they?

Keep in mind that the US is the only developed nation, wehere the law mandates exactly zero days of paid vacation.

You may get the odd holiday off, but for the rest you're totally at the mercy of your employer.

I believe it's also one of only two countries in the world that doesn't have standard paid maternity leave. It's bizarre just how few rights American workers really have.