My bathroom is close enough to the kitchen I would say fine and put him on the potty and leave them till he either poops or meal is almost done. Then return him to the table to face his now cold lunch while others finish, are washed up and go to play.

It is entirely possible he isn't wrong in the sense that if he is holding it and it is a hard chunk right at the edge - sorry if too graphic - then it likely does hurt when he sits at the table on the hard chairs and thinks about it. Also when he starts to eat it starts the digestion process going and that will include the bowels working which is why kids often poop after eating. But for sure it is a stall tactic on eating or a learned behaviour where he can have undivided attention. If parents are sending him to the potty on his own rather than disrupt their meal then they likely don't know if he goes or not or if he just plays so may be trying it at home too.