Jackson has a new joke.
I use the word "joke" loosely.
The bit goes like this.
Were all three sitting together, maybe playing or enjoying a meal and Jackson tries to get your attention.
"Mama! Mama! Mama!"
... and the joke is that once he gets your attention he screws his mouth over to one side, juts out his chin, raises his eyebrows and makes a face as though he just didn't quite catch what you were saying and responds:
and then he bursts into giggles.
Get it? He's called you ... but now ... he can't hear you.
That's the joke.
He finds it even funnier when you play the joke on him as well.