Regardless of why the son is too tired the Op shouldn't be punished for it

You're right that we can't really know what illnesses a person has (though the way OP worded the FML strongly implies she KNOWS it is just laziness), but...it's still wrong of the boss. If he wants to hire special-needs workers who he knows are extremely unreliable when it comes to hours, then he should make sure to have enough employees to easily fill those gaps. Not ask the already overworked person to fill in his shifts at the drop of a hat.

or like some said before, his son can be working part time after even highschool and might be tired

ME is not special needs so that is just ignorant right there, the dad probably is letting the kid do some work so he is earning a little bit of money in a less stressful environment so as not to cause any health damage

Who says she is being punished? She is working extra hours yeah but she is being paid for it and I bet that other workers are asked to cover some work at other times and OP is the only one whining about having to do her job

I don't get why this is being down-voted. I know it doesn't make the boss's behaviour any better when he over-works the other staff because of it, but I have friends with ME or similar conditions that give them low energy, I'd hate to think people who don't know anything about their situation write them off as lazy because they're some of the most determined people I know.

I wasn't really referring to ME specifically. That's why I said "who knows what illness he could have," as in there are multiple diseases out there, other than ME, that we can't necessarily easily see but that will require special treatment/allowances/understanding (if that is more politically correct than saying "special needs" to you). And it doesn't matter if the dad wants to let his son earn a bit more money in a stress-free environment. If that's the case, it shouldn't be at the expense of making his other employees work in a MORE stressful environment. And we know this is not fun for OP, because she is complaining about it. If she needed the money and was super happy about the extra hours, it wouldn't be something to complain about, now would it? I have worked for a dude who hired lots of students, even though he knew they were often unreliable when it came to hours due to exams or just being tired out. But you know what? When someone cancelled a shift, he made sure to hire enough people that there was always someone else he could call in. Or he'd often come in and fill that shift himself, since he knew HE was the person who hired someone he knew would probably cancel last minute a lot. And the worst he's ever done is ask me if I'd be ok with working a double. Not ORDERING people who are already working long hours and really tired work, at the very last minute, that they better be ready to wake up 4 hours early tomorrow.

The way it is worded, it sounds like OP's bosses son does not have an "invisible" disability and is just lazy.

I don't think you understand than an invisible illness is INVISIBLE, mate.

I notice every time that some goes against an fml like this and tries to give a reason for the occurrence we are downvoted, it doesn't make sense really to downvote someone who tries to explain something but everyone is so quick to judge