We are teaching our children the whole "when in Rome" thing... when they are at their grandparents they will bow their heads and hold hands or whatever is expected and be quiet during grace. You think it's all hogwash? Great! Good for you! This little ritual makes your grandparents happy, so be quiet and let them have it.
As for the teen in question, I would try to not make it a fight, if at all possible. I'd prefer it, personally, if he would sit, quietly while the rest of the family did the ritual/ceremony/etc and then ate the meal with us. Being polite is a completely separate matter from personal faith, IMO.
