DS is 2.3 and weights 12.5kg. He is also tall for his age and height.
He is currently in a Stage 2/Group 1 car seat (Britax Eclipse). The top of his head comes very slightly over the top of the seat, which suggests to me that he needs to go into a Stage 3, although he is well under the weight recommended to move up (15kg on RoSPA webiste).
I have tried him in a friend's Stage 3/Group 2 car seat and he is tiny in it. He is so skinny that the straps barely tighten over him and if I were to use one for real I certainly would not trust him not to undo the seat belt!
What to do?