honestly, have an exciting conversation with him, even if it’s a load of rubbish, talk about something that he will engage with, be over dramatic when he tells you something - be interested in him and he will sit there. He knows you r scared of him, he knows that you hate eating with him and he knows you would rather be talking to strangers on your phone than him.
He was trying to engage with you by saying the bird and squirrel are our pets now and you fobbed him off and told him to shhhhh, and kept filming Isla and a magpie. He was starting a conversation with you, wow Lachlan!!!! Imagine we had a magpie for a pet? Where would he sleep? What would we feed him? Would we take him for walks? Easy.
stop moaning about your son and listen to the advice people here are giving you. The answer should never be to strap him in his high chair, how embarrassing for him. And the fact that you share embarrassing him with thousands of people is another level.