When my kids take their lunch, they either buy milk or take a Capri Sun drink that has sat in the freezer all night. By lunch time, it is no longer frozen and a plus is that their lunch stays cool. You could fill a container with the drink and put in the freezer overnight. If he has an early lunch, put it on the frteezer door. For a late lunch, put it inside the freezer. It will freeze more solid. You can freeze milk with no effects.