when to turn my child's car seat?

I know that everything I read says wait until your child is 2, if you can, to turn their car seat forward facing. But how can you really put an age on it when kids are all different sizes. My son is very tall. He's 10 months old now and he is 27lb. And not an overly chunky 27lb, he's just a big boy. Last time we were at the dr they had him in the 99%. Even though his infant car seat says it goes up to 40lbs, it just doesn't feel as sturdy with him in it now; like it used to. So I ordered a convertible car seat. But If my child is the size of a 2year old, should I turn his seat forward facing? Thanks for your advice mommas!