I have an almost 10yo still in a booster, he is not tall enought yet to meet the 4 safety points for sitting in a car w/out a booster. Honestly, I never really paid attention to the law; I think it provides the bare minimum for safety and there is other, better info out there to determine the safest way to transport your kids. Height, weight and age are too general IMO.
Here's a list of the 4 safety points to determine whena kid is old enough to ride in a car w/out a booster, I doubt that there are any 6yo's that meet these points.
It may be a pain for you, I think it's important to keep belting him in until he's able to do it on his own. When my oldest was 6 I had a 1yo and an infant..I remember the hassle of using the car all to well ;)
Here's the checklist for riding w/out a booster for when your 6yo is older:
They are tall enough so that their legs bend at the knees at the edge of the seat; and
They are mature enough to remain seated with their backs flat against the back of the seat and not slouch; and
The lap belt sits high on the thighs or low on the hips (NOT on their tummy!); and
The shoulder belt crosses the shoulder and chest (NOT on their arms or neck!)
Some organizations will also give limits like 80 or 100 pounds, 4'9" in height or 8 years old. These are rough guidelines, not absolute limits.