I would let him cry if he wants to. You could turn the radio on for some relaxing music or if you don't want to hear him scream put on some Led Zepplin. We all went through screaming kids in the car. It's part of being a mom as well as poopy diapers, having them spit on you in the grocery store. Being a mom means work work work work work. Not having a free moment and wiping spit, poop, vomit and pee 24/7.