Car seats for 4 year old

Because I can not find info online im wondering if anyone happens to know... I have a very big 4 year old... He is 3 1/2-4 feet tall, and weighs 58 pounds!! He currently sits in a booster seat using the cars seat belt, with the backrest attached to it... But he hates the backrest and I'm wanting to know the requirements for being able to take it off, and just have him sit in the regular booster.... I live in California.... Thanks