DS1 is 135cm tall but isn't 9 yet for a few days. According to the site I have looked at he doesn't need any car seat now, just a seat belt. He isn't very tall for his age IDT so 135cm or aged 12 seems a big gap.
I think he is still okay in his seat, he doesn't look too tall/big for it, but is there a danger to him being still in a seat when he doesn't need it? He is in a rodi xp.