Wow. I think it sounds like you need to let mom know that she and you need to be on the same page with how her son behaves andhow to deal with it when he misbehaves. He's screaming for attention and getting it. if mom allows certain behaviors that you won't most disipline techniques will faulter.
I would give her two weeks to asist with teaching him tantrums are not ok--at least to start. i would let her know that you two can re-evaluate the situation in two weeks and go from there. if things improve, they can stay. Not many places will tolerate that kind of behavior.
Now, if you need the money and are afraid they will leave if you mention any type of change to them (and i really don't advocate keeping a child on strictly for the money), you may just have to deal with it. respect is learned through what you will/will not tolerate. if you tolerate the behavior of both mother and child they will continue to behave that way.
good luck and take care of YOU, too. Vacation or not...you're child needs YOU more than he needs a vacation. Plus, he's wittnessing some pretty unruley behavior which isn't healthy either. JMHO
ps. after all that getting beat up on thought you could use some (((hugs)))
take care
