convertable seat for a tall but skinny guy

[msboog]

I need to get an additional booster seat for the van that takes my 4 year old son to school. Right now he uses a Century Accel but in the top slot the straps are just at his shoulder. We have a Britax Advantage in our car which I love but I didn't want to have to spend quite that much money again. My son is about 37 1/2 - 38 inches but is skinny and only weighs about 27 pounds. Can anyone recommend a good convertable seat with a high height range?? Thanks.

Robin in Mass.