At six, if he is mature enough to sit correctly 100% of the time- no wiggling around, moving the seatbelt, even flopping over when he falls asleep (you can teach them to look up when they get sleepy, so their body remains upright)- and meets the height and weight requirements of the seat, he should be fine in a booster. 
The maturity factor is one thing I think they should have mentioned as well- some 5 year olds might fit the size requirements for a booster, but they might not be mature enough to understand they cannot play around with the seatbelt while the car is moving.