"He has stayed with other people in the past" and mom has always worked the night shift....The kid is messed up because he does not have consistency, and he is angry at the world because he does not have his mom, and he is angry at you because, well, you are not his mom.
This is NOT an excuse, but it is an explanation.
He does need to change the attitude. But it sounds like his mom, who is working hard to give him a decent life, also does not get respect from him -- if he respected her he would listen to her. I feel so sorry for her because she may feel she doesn't really know him, considering their schedule.
I would talk with her again, at a time when he is not around to hear it; at a time when she is not picking him up in the early morning hours; at a time when she is not due for work soon and is in a rush. See if you can arrange to meet with her when she has other "coverage" for him during a day she's off work, and talk to her as a friend. He is working up some anger issues here that will follow him and get worse, not better, if she doesn't get him some consistency and help soon. I would ask her if there are also issues with his behavior at school (I'd bet it's highly possible). I always advocate parents talking to a school counselor at times like this and she can consider that route. Focus the talk on helping him, not criticizing him. She already knows he behaves this way; make your talk with her about "How can we help him grow up happier and a better member of the household" rather than "He's being rude again."
But she also needs to set up some consistent systems of reward and discipline--"If you do X, the result for you will be Y." She is not there to see and hear the behaviors, which is very tough, and he sees you (and his other nighttime caregivers) as "Not my parent and not my boss." Possibly she can write out a simple contract with him laying out the specific behaviors he will use and the rewards if she gets a good report and the consequences if he gets a poor one -- and the consequences should be spelled out specifically (you will lose a week of TV time, you will lose all Wii for two days, you will lose...) etc. The tough part is -- you will have to report on him and you will have to enforce her discipline when she is not there. If you are going to continue as his caregiver at night, you and she need to work out together a clear system and ensure he knows you are acting on her behalf.
It sounds as if he has had to shift from person to person. He likely feels he doesn't really have a home and he is always the tag-along to other families, so he is acting out. The perfect-world solution would be for her to get a day job, but that is not necessarily realistic (if it's in the range of possibilities, though, help her hunt!). Meanwhile you will have to "become a discipliner" as you put it, with her complete input and agreement, or he will continue to get worse. IHe is too young to see that he's really upset and unhappy. You are the adult and old enough to realize that -- and to be there to help him through it with consistent discipline. (Which does not include spanks or swats, by the way -- if things went bad between you and her, that could end up as a legal disaster for you. Stick to taking away things he values.)
Be sure he has some regular chores at your house, as if he's a member of the household. Be sure he is thanked and praised whenever possible (even if he's doing something you consider just normal, if it's something he hasnt' been good at --like saying thanks--praise him for it). Be sure he definitely has a firm routine at your house every single night so he knows what to expect each hour -- snack; homework time; you sit with him and check homework is done; reading time; ready for bed time; reading in bed; etc.