Where should I put my 6yo son in the car if he is really small and light for his age?

Booster seat. The recommendation of the American Academy of Pediatrics is that a 6 year old be in a booster seat in the back seat of the car.? All children whose weight or height is above the forward-facing limit for their car seat should use a Belt-Positioning Booster Seat until the vehicle seat belt fits properly, typically when they have reached 4 feet 9 inches in height and are between 8 and 12 years of age.
Rear seat, harness . Older toddler and child weighing 20-40 lb can be placed in the rear seat in a forward-facing child harness seat. Make sure that the child is buckled into the seat correctly.
Rear seat, harness . Car safety seat products now include forward facing harnessed seats with upper weight limit of 60-85 lb. Harnessed seats are the only device providing significant protection in roll over & side impact collisions. Unharnessed boosters are adequate for most frontal impact collisions, but you don't get to choose your crash.No kid under 100lb/12y should ride in front.Middle/passenger back is best.