Parents with dry ice experience, how long will it keep cold?

Another thing, is that you mention using a cheap foam cooler. Dry ice is carbon dioxide and in this heat I am sure he will be driving with windows up. Alot of people do not know that you can be asphyxiated with dry ice in a small space and not even know it until its too late. Be sure and use a non cheap cooler and make sure it is air tight and sealed.