My son is very small for his age, and at 6 months or so we got him the Britax Diplomat convertable car seat--rear facing of course-- and it works very well for him. It has a a removable pad for trunk support and an adjustable headrest to raise as they grow. It has been amazing, and it's very durable!!! Like I said, he's always been small for his age, so at 6 month he was only 13lbs...
