My son is 9yo, 54" tall and 68 lbs. He is not required to use a booster in Colorado, but he so thin that I think he should. When in the back seat, there is so much room side-to-side that it's easy for him to move out of a safe position (he wiggles around a lot) and if he falls asleep it's worse. He's tall enough that I don't feel he needs the height of a booster seat, but I'd like to find something to support him side-to-side in the seat and back and to position the belt correctly. Currently I've got him in an Evenflo Big Kid Deluxe, but he'll be too tall for it soon and it almost touches the roof of my car. Any suggestions?