I think because he knows you won't/can't remove him completely, and that you'll be mortified so more vulnerable is possibly the reason why he ramps it up.
I get the not getting him away, but a shock tactic of leaving the meal/event once and apologising for his behaviour might just show him that you will follow through.
I think he sees the chink in your armour and is using it to the max.
You can be consistent and carry through your threats longer than he can.
He's old enough to understand, albeit too young to be emotionally able to communicate his issues, but that's no excuse to ruin your holiday or that of your family.
Battle of wills, perhaps, but one you have no option than to win. In a few years time, when he's well practised in this stuff, you won't have a prayer of pulling him back around.
I don't envy you, I really hope you can get through to him. It's a miserable situation for you and for him.
When he's calmed himself, does he cry and seek reassurance?
Mine used to kick off a bit (only with mw though, so difficult tbh) and once i'd got angry with him he'd cry and it'd turn out he needed cuddles/affection.
He'd play up if there were others to compete with.
I told him that if he wanted attention or cuddles to talk to me, but not blow up to get them. Eventually it worked...
5 is shit. It passes though!