when i had my 500d i thought it was pretty noisy from 400 iso onwards, although usable.
at 800 iso it starts to get a bit iffy and 1600-3200 are unusable imo.
in terms of image quality, my own research led me to believe that the 550d is a very decent performer, better than the 50d and 500d at equivalent iso.
if pure image quality and iso performance is what you are after, it would seem to me that the 550d is the better camera out of those you listed.
if fps, build quality and other pro features are what you need, then the 40/50d seems the sensible option.
given this is for your son, i'd get him spanking new 550d. i'm sure he'll be chuffed to bits with it.