How do i make it so he cant undo his seatbelt?

Put the release button facing him and make it secure. He won't be able to see the red push button/release button so he may be frusrated and throw tantrums (if he's anything like my son) so try taking him on a few car rides with the button hidden before your road trip so he can get used to the idea that he won't be able to get out. Hope that helps, good luck!