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You can booster style car seat with the 5 point harness.my tall 6yo is in an infrsecure booster and still has heaps of room before he uses the seat belt
I thought they legally had to stay in the car seat until 4 & then they can go to a booster if they reach the height criteria after their 4th bday - at least that's what we were told at our local baby bunting when we went to buy a booster- thats nsw anyway. Google it.
It should be by height, but you'll need to test them to make sure he reaches the markers, they might be different for each brand. If you have a kidsafe store nearby it would be worth a visit for advice.