1) Not telling your family that the work environment is lousy was a mistake. But there's no reason why you can't tell them that now
2) Can you work part time? Fred doesn't see things in any way other than black or white. Part-time offers a gray area that no one seems to have considered. Never mind if no one else works part-time.
3) Do you make money in any other way? Your family may be not so subtly telling you to start bringing in $. It doesn't matter if they don't "need" it. That's not your place to tell your parents etc. what they need and don't need. But you
need to start making money at some point.
Even if you're a fabulously wealthy trust fund baby, you still need to establish a salary history. If not there, then somewhere else.
4) You have already volunteered at this magical place. It can already go on your resume. So getting paid work there would make little if any difference.
5) Has anyone talked to Fred's boss? You shouldn't be yelled at, at work. No matter what you do. And he obviously can't put together a simple schedule. His supervisor needs to know this.
And speaking as a supervisor, I can tell you for a fact that if you don't tell management that there's something wrong, management will assume everything is fine. Nothing will change unless someone makes some noise.
You don't have to be mean or suggest that Fred be fired. Hell, you can even send a note anonymously. Tell management.
6) Fred clearly doesn't want to be bothered with details. He doesn't want to have to remember a lot of stuff.
So, has anyone actually tried putting stuff in writing for him? If you agree to work even part-time, Put. It. In. Writing.
You will then have something to point to if Fred makes you work all day (if people are being paid for all day work, then they're not being harmed by his practice of keeping them there, no matter how much your coworkers complain. If they are not being paid then, again, they need to go over Fred's head).
Something in writing is also something you can show Fred's boss as support for your position.
7) And finally: a whole 6 months with no days off! Quelle horreur.
Your schedule outside of work is not Fred's concern beyond whether you can work.
Plenty of people go for as long or longer without taking any time for themselves. This complaint is not a legitimate one.
Fred and his manager will likely tell you to suck it up, Buttercup, if you make it part of your communications about this. So, don't include it. You want any complaints to senior management to be short, impactful, and to the point.
Time to get started on communicating better, all around.