Yes, I would discipline him, but the main thing is getting his parents to buy into it so there's consistency. They wouldn't fault you for doing the right thing, would they? Clearly his young mom was upset but did she have any advice?
difficult child acted up with-my Dad once, and Dad, then 81 yrs old, grabbed difficult child by the arm and shouted in a deep baritone, "Cut it out!" Gosh, how I'd love to have a voice like that! We all jumped when he did that to us when we were little. Yes, difficult child behaved after that. He pouted, but he behaved.
You said, "He is an only child, used to being prince." It sounds like he was fine with-you because he was alone, and he didn't like the competition of Grandpa and the girls and he has no other way to express it.
When he gets like that, you have to remove him from the situation and remove yourself, too... otherwise he's getting attention, and he doesn't care whether it's negative or positive attention.
(Sheesh, this is so easy for me to say... wish I could have been consistent with-my own advice when my difficult child was that age! :grin: :blush:)
Good luck.
