MB here. My 8yo acts like your 9yo NK. I have decided to start insisting that she tolerate losing and winning if we are going to play. That means, no gloating, no whining, no complaining. If she starts doing any of those things, we put the game away immediately and I explain exactly why.

I'm not mean about it, but very firm. I tell her, look, life is about winning and losing and you're still learning how to handle those things. That's ok. But it's no fun for others if you scream when you lose or gloat when you win. So we will not be playing when you're doing those things. She definitely melts down when I put the game away, but to me, the meltdown was going to happen regardless, and at least she'll learn something from it if I hold firm.

She has not successfully made it through a game without a meltdown yet. But she's at least motivated now to try to manage her emotions because she knows we won't play with her if she's being an a**hole. I give her a lot of grace because she's learning, but hold firm on that boundary. Because honestly, boundaries are what allow me to enjoy my kids' company. And I'm not doing her any favors in life if I don't teach her to be cool and not freak out over board games.

So I definitely recommend setting boundaries, or else not playing games at all. You're not his parent so you have less responsibility to ensure that he can tolerate losing than a parent would (not your circus) but I don't think it teaches a good message for any caretaker to suffer through horrible behavior and not set boundaries. So playing games and just letting him win and gloat ... that's not reasonable to be expected to do that.