I use Graco turbo booster and it fits perfectly under the head rest with a rubber seat protector mat underneath. The Graco has a vertically adjustable headrest which makes it easy to fit under the head rest and very stable in the car.Depending on your height, you can fit almost any carset in the backseat. I currently have the Graco 3-in-1 toddler --> booster seat and it works well. The secret to fitting the seat properly is to remove the headrest!
Yes he is808elise, s that your son? he is so cute