IMHO he will be restless, and the tables are kind of close together so you really wouldn't be able to let him run around much. At the beginning they let kids come up to dance , but only for a short time.
Now at 'Ohana they have periodic coconut rolling contests (kids roll the coconuts around with brooms - your 6YO and the little one plus mom or dad could do this) and hula hoop contest, plus singing. It may be enough to keep his attention. And the meal doesn't have to be 2hrs.
I'm just saying the price for the Luau won't be worth it if you end up leaving to let him run around the gardens outside the show.