Mike, you are from Europe right? I understand that generally they are more civil about things like this there, so through my eyes, this is Black and White. Functionally, he has very little recourse, and its probably much more useful for him to think about getting what he needs however HE can go about it, without using something like a "labor board."
His contract is more than like this. At will employment. Boom. Done. This is how many blue collar jobs are across the country. Forgot about it in a ski town. If he goes to his boss (who seems to not care about the employee's wants and needs already) and complains, the boss goes "too bad." If he wants to make grandiose threats, thats fine, but now he is fired, because he is in an at-will position with tons of other people ready to take his hours. Sure, thats illegal right? Doesn't help OP though, cause the one in a million chance someone wants to investigate (GASP!) workers-rights violation in a ski town valet position won't happen for him. Either way, he needs to eat tonight though, and a year long investigation wouldn't help him with a paycheck anyhow.
I'm just trying to be realistic, not aggressive. Sorry, but some of the ideas of how a workforce actually operates around here are laughable.
OP needs to be proactive about getting what he needs, or just suck it up. Working overnight is not the end of the world. I do it, 7pm to 7am.