At what age did you do the switch from car seat to booster seat?

My son is currently 5 and a half and his father has him in a booster. I told him I disagree with it and think he is to young. He wants to sit in a booster I'm just not sure he is ready. I bought one just to see how he fit in it and he was so happy. I just don't know it's safe.

*Note: he is very tall for his age. His height and weight is in the 110%. He is about the size of an average 7 year old.

I'll get a picture of him in his when I pick him up from school.

Please! Any advice is helpful! TIA

This was the best picture I could get (his dad took it)