Is it okay for my 6 year old to move to a booster seat?

He’s currently in a 5 point harness car seat, he fits fine in it, but we’re going to have to shuffle the car around to fit the new baby’s car seat until we can upgrade our vehicle, and the only option is for 6yr old to go to the back row, which has much more compact seating, and the large current carseat might not fit, or if it does, it won’t be comfortable.

He’s currently just barely 4ft tall, and weighs around 46lbs.

Car safety is SUPER important to me, so if he’s ready for a booster, what should I get/look for/etc? This is all new territory for me 😅