Booster seat recommendations

akaoct2005

I know these questions are asked a lot...my kids are too little still, so I don't pay attention, but my sister asked me to ask the gurus:)

My nephew is 5.5 and just 40lbs (and he's short, but i don't know his exact height).  He is currently in a britax marathon, but my sister thinks it's time for a booster.  Any recommendations?

Thanks!