I have the same expectations for all children regardless of age ~ you do not have to eat but you have to sit at the table and keep us company ... company means you are supportive of your friends need to eat without being silly just sitting making quiet conversation ... I would remove his utensils and food / plate if he is playing with it. I would worry however that a time out might back fire in this case because 'removing him from the table' is what he ultimately wants he might not care if it is to sit on a chair by himself as long as he is not having to sit and watch them eating.

Have you had conversations with mom before about how is meal time handled at home or that he has been having issues before you sent today's email ~ how do they handle this issue at home do they share this expectation?