Believe me, I get the point. Others aren't listening to or responding to mine.
He IS being taught to sit still. It's not a "one and done" lesson, though. He needs a lot more support with boundaries in this regard than others. Which means doing it, repeatedly, until he gets the message. Even if that includes bumps in the road where he needs to be escorted to the play section.
Think about all the possible things a child learns and develops - sleeping through, weaning, learning to talk, learning to walk, eating vegetables, brushing teeth, potty training, dressing, care routines.
There's a HUGE natural variation in all of these things. Do you agree with that, or do you think that all children learn all of those things at the same pace? If you agree with that, why do you then think that sitting still all through a meal is different from other forms of behaviour or learning? Did you teach your children to do those things by showing them once and they just magically understood it?
Incidentally, I don't mind at all the posters who think that if my son isn't ready to sit still in a restaurant then he shouldn't go. We might disagree on methodology, but they at least realise that different children have different dispositions.