child car seat requirements

Car seats and kids in the USA.?
My family are coming over from the UK next week and my little brother is coming over he is 6 years old just under 4feet tall. What are the requirements concerning child seats in cars?
I know in the UK he has a “booster seat”, what will he need here when hes riding in the car, does he need a seat can he sit in the front of the car etc?
under 4′10″, you need a booster seat. there is no “age.” I started riding up front at 10 because I was tall enough.
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