Yep. Law is 6 OR 60 pounds. Their is legislation to change it to 8 AND 80 (and of course exemptions for those whom the law is not practical, as there are now)
Screw the law though. You need to do what you think is the safe thing to do for your child. Is he tall enough to sit in a regular car seat with the shoulder strap sitting properly at his shoulder? If not, I would definatly bring his booster. I would guess that at 4'9" (which OMG is a tall 6YO!) he his tall enough to be properly restrained in MOST cars, but seeing as you probably don't know the exact seatbelt arrangment in the rental car I would bring along the booster just incase it's needed. Even in your own car though, if the shoulder straps hit him below the shoulder when in the booster (as would be the case in alot of 3rd row minivan or SUV seats), you need to go to the Dr and get him an exemption. He's a VERY tall 6YO and if that is the case would be safer in those seats without a booster.