Just because he's got the seat slammed doesn't mean it's too big. We don't really know if it's too big for you until we know what your cycling inseam is. See here for how to measure:
http://www.competitivecyclist.com/za...der=M&units=cm

FWIW, I'm 5'8.75" and I comfortably ride both a 56cm and 58cm frame with a cycling inseam of 32.75".