when did you switch ur baby to a booster seat?

Esthela
My son turns 9 months in a couple days. I bought him the Graco Nautilus 65 lx Conley about two months ago. I haven't switched him because I felt he was still small at the time. I saw a friend who son's is the same age as mine and she has him sitting in a scoop booster already. I feel like I bought him a booster to big for him. I went according to the weight and he sits in it perfectly. It is straight and I think if he falls  asleep he will be uncomfortable....... anyone have their 9 months sitting in a booster seat??? 
This is two months ago. He sat perfectly into it but he seemed too straight up. My question is, would it be ok to switch him?