Some cabs do carry child seats
As long as your grandson uses the restraints available and doesn't sit in the front, you will not be breaking the law
Personally, I would not hire one and feel your g/son would be safer on a cushion with a seat belt, not the lap belt by the way, in the rear of your car. You wouldn't know how many knocks a hired c/seat had had and I don't think there is anywhere that does hire them out anyway.
As a one off this is fine, but if it becomes regular you can buy booster seats in Woolies for �10 - why not buy one anyway? Cheap and it will last until your g/son is of an age whereby he won't need a car seat.
ojx