We currently use a BeSafe Izi Combi beltfix ERF seat for a heavy almost 3 and a half year old who will soon be reaching the weight limit for the ERF seat. This may sound dim but what should we be looking at next? Does he move straight to a booster cushion and the adult belt or is there an inbetweeny stage seat? I spent so long researching rear facing but don't have a clue about the next stage!