Ok, so DS was occasionally slouching in his booster today on our drive to ocean city. It was about a 4-5 hour drive since it was snowing. His normal seat in a nautilus but the chest clip is broke so a booster was the only option. The booster he rode in is a first years b570. Was his slouching because he was just tired and not used to sitting in a booster full time? He rides around town in a booster with DH, my MIL, my mom, my babysitter and anyone else that drives him. He rides in the nautilus with me and the van that is the trip car. He has also most outgrown the harness in the nautilus so I don't know what to do. Should I try to get him a frontier85 or is it just practice that he needs in a booster? He is 6.5, 52 pounds the last time I weighed him and about 48 inches tall.