3 year old won't stay on his chair for meals

One of my twin 3 year old boys can't seem to sit on his chair for a meal for more than 10 seconds at a time. I'll ask him to go sit down and he will and then take a bite and he'll get back up. He is still eating but he might be eating from beside the table, from under the table, maybe standing on a chair, maybe laying down on three chairs, but just sitting down seems to be impossible for him for more than a few seconds. His brother isn't perfect but generally stays there for most of the meal. I'm hesitant to lay down the law too hard because: he's 3, he's a boy, he's not directly defying me... just seems to be incredibly restless. Should I be letting it slide a bit in the interest of preventing the conversation at the table from being "Hey! sit down!.... sit down now!.... NOW!" over and over? via /r/Parenting http://ift.tt/2ni1ZDJ