I would tell your grandson you know he is putting his lunch in the bin as it's obviously worrying him. He is only 10 and he shouldn't be having/needing to do this. Tell him this is not a problem, he should just tell you the reason why he is doing this. Tell him you can solve the problem together. It may be a case he is too busy playing football at lunch time and then he forgets till home time? Tell him to put the sandwich in his pocket and eat it in the playground. You don't know the problem till you ask. If it is not simple and you feel something is really wrong then tell his dad straight away.