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.