All of the above ideas are probably true, but I am going to disagree with how previous posters would deal with this issue. I think that mum or dad need to know right away. They can watch out for any other food related issues at home, just in case there is more to this than others have suggested. If they feel that there is any eating problem, it should be dealt with as soon as possible. If your talk to your grandson first, and if he is hiding food, he will get clever about how he hides it and any issue will be masked. That said, it is most likely a desire to get out to play that is resulting in lunch not being eaten. Is your grandson hungry during his time with you? I would be famished if I had passed on lunch.