Car seat question

[Peanuts33]

What age can my ds just sit on a booster seat without the back bit. He looks really uncomfy with the back bit on now.

He does sit on a booster cushion occasionally but that's only when I'm ferrying other kids around.

He is 6 and average height for a 6 year old.

hope this helps...
www.childcarseats.org.uk/types/index.htm

My DD is 6 and is more comfortable withou the back bit but I have bought a car seat belt router as want to make sure seat belt sits nicely on her shoulder not hits her in neck.