Remember that older kids should usually be in a booster seat until they are about 4-feet 9-inches tall and are between 8 and 12 years old, when a car's regular seat belts will usually fit them well.
Signs that seat belts fit your child include that the shoulder belt fits across your child's shoulder, does not go on his neck and you don't have to put it behind him. Also, the lap belt should fit low on his hips and not on his abdomen.
According to the latest car seat guidelines, kids should move to a belt-positioning booster seat when they reach the weight and height harness strap limits of their forward-facing car seat. Again, the move to regular seat belts should not occur until kids are "old enough and large enough" for the seat belts to protect them properly, which usually isn't until they are 4 feet 9 inches tall (57 inches) and are between 8 and 12 years old.