10 year old hitting/bullying me when parents are not around

I started nannying for 4 kids about 4 months ago and our relationship has only gotten better and better until recently.
During one of the family trips, the oldest kid got really close to me and it continued for about a month after the trip. About 2 weeks ago, he started kicking me on purpose, sometimes to make it less obvious, he pretends he accidentally kicks the ball to me but really hard. The maid noticed it and told him to stop kicking and dad scolded him for it, he then switched to using his hands to hit me instead. He only does this whenever his parents aren't looking. He also says mean things to me on purpose, for example I was picking him and his siblings up from school and he said loudly 'ugh why is she the one picking us up!' And 'I don't wanna sit in the front because she is driving'. And many times blame me for things I didn't do. Like saying it's my fault that he is late for school etc. also calling me names like 'bitch' 'weirdo'. Telling me to go back to my country, why are you even here, leave this house etc. one time I was colouring for the youngest child in the restaurant and he stood up and went out of his seat that was blocked by 3 other people just to come near me and ruin my drawing/colouring. It's mentally exhausting and it hurts a lot when he hits me. I've tried asking him what I did wrong but he wouldn't answer me. His parents as well has noticed it and asked him what's wrong but he wouldn't say a word, or maybe he doesn't know why or what to answer. What can I do to stop this behaviour, it is the total opposite of the him I was with during our family trip. Sometimes I think of quitting because of his behaviour but his mom said that she wishes me to stay. I suspected that he wants my attention because I'm not playing with him as much as I used to during the trip, because I'm busy accompanying his younger siblings that needed the help more. But when I go near him, he swears at me, call me names and telling me to leave the house.