had no idea what to do πŸ˜±πŸ˜“

Mo β€’ Married 9/24/16. Mommy 7/22/17 ❀️
So I'm a nanny of a 6yr old boy and 4yr old girl. I've been with them for 3 years now. We are basically family and we spend every day together! We all love each other very much. 
That being said this past year the 6 year old was obviously in school all day so it was just me and the girl (who just turned 4 the other day so she was 3) and the boy only got to see me an hour every day but the girl and I spent all day together. Obviously that effects the relationship and even though he still loves me he felt left out and I understand. I always tried to make him feel wanted and included when the 3 of us were together again. 
Now it's summer and I'm 36 weeks pregnant and he just isn't behaving in the slightest. 
He refuses to hold my hand when we are in parking lots or crossing the street. 
I picked him up from camp today and when he got in my car he said that I "boss him around" and he doesn't like it because he's "the boss" (hahahahahaha nope not even! Nice try tho kid) 
But then at the playground πŸ€¦πŸ»β€β™€οΈ
He was playing at his grade schools playground with other school mates (their camp is at their school and after it gets out they all play for a bit)
He was chasing this little girl with some other kids. Harmless fun and if it wasn't the girl is 7 and she can absolutely say something to them or go to her parents who were both there. 
She didn't want to be chased anymore but was still running and laughing and fighting with the boys so none of us stepped in. 
Finally she goes over to her dad (who as it turns out works at the grade school they all go to) and says "daddy these boys won't leave me alone" and he tells her to stand up for herself and tell the boys to stop. He says "you asked them to stop very nicely and they didn't - you don't have to be nice anymore. Tell them exactly what you need them to stop doing, mean it, be forceful" 
Great advice! And she did it and it worked. On all the boys except mine.... 
he continued to bother her, poke her, take her sandal and get in her face. (All in a matter of seconds) 
Now mind you I'm watching all this on the other end of the playground trying to watch the situation play out before I interrupt (cause kids their age really do need to figure out how to work on things themselves) but I start gathering our things thinking it's time for us to get going and I'll make my way over there (slowly.  Thanks pregnant waddle πŸ˜“)
Now the dad is pissed and he looks at David and says "I work here and I know exactly who you are and you need to leave my daughter alone or I'm calling your parents and telling them you're harassing my daughter!"
Totally fair! I'd be furious too. Nobody should treat someone that way. And instead of backing down my kid gets in this adult mans face (chases after him actually cause the family had started to leave) and he says "oh yea?? You know who I am? Who am I then! What's my name??" And gets super sassy. I'm dumbfounded! I can't believe he thinks he can talk to any adult like that!(or anybody for that matter but a grown up??! The balls on this kid...) so I get his sister and I'm just like alright we are leaving NOW. And I plan on talking about it with him in the car but I'm so furious with him and he instantly becomes sassy with me (refusing to hold my hand. He glares at me with these like evil eyes. And then In the car he tells me I'm bossy! Hah!)
And I just feel like I'm too emotional, tired and too close to the situation to come at it from a fair and level headed perspective so when we get home thankfully his grandpa is there and I tell the grandpa about it and now he's putting the boy to work on the house all day as punishment (yard work, painting etc - which is why grandpa was there anyway) 
But I just don't understand what would possess this boy to act that way!
I should've stepped in sooner. Apologized to those parents. Scolded him. Yelled at him! Idk!
He isn't my child so I don't ever want to over step my boundaries but I know if he WAS my kid I'd whoop his ass no fucking questions. 
Do you think it's a phase? Are 6 year olds just like this? What is going on? What can I do next time? I'm going to talk to his parents but they're both out of town until Thursday πŸ€¦πŸ»β€β™€οΈ but this kid doesn't listen to me anyway! So what good would it even do?? In the fall I won't be watching him like ever again so I'm wondering if I just throw in the towel and say ya know it's not my problem I barely spend time with him and I'm clearly not an authoritative figure anymore. This is my first family I've ever nannied for and I know if I wasn't pregnant I might take him on but he just stresses me out so much πŸ˜“πŸ€¦πŸ»β€β™€οΈ
My MIL thinks he's jealous of the new baby? Idk. I'm sorry ladies I'm all over the place I'm just so dumbfounded.