He's long for his age, and the feet are well past the edge now. Weight is still inside the limit printed on the seat. The shop says move up to a convertible seat, but obviously the shop would say that. Did you go by length, weight, or just how it looked?
Our daughter is nineteen now, so my memory of car seats is limited. What I do remember is that the manual gave a height limit as well as a weight limit, and the point was whether the head was below the top of the shell, not where the feet were. Check yours for both before buying anything.
We moved our daughter up early because a relative said her legs looked cramped, and looking back that was the wrong reason. The feet bothered us far more than her. One practical thing: the convertible seat we bought was too big to fit rear-facing in our small car. Try it in your own car before you pay.
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