I'm a bit confused! My son is 2 and a half years old and is roughly the same build as our nearly six year old son, at the moment he is squeezing into a britax first class si car seat. I've searched the web for advice about car seats but cant work out if my son will be safe in a high back booster chair?? it says weight suitable from 15kg which is roughly 2.3 stone and my son is roughly 2.6/7 stone but the age on the highback boosters says approx 4 years?! any advice please?