I believe there is a standard and bantam variety; the standard being small for a standard bird.
Is he doing showmanship? I wouldn't be too worried about having a light standard bird (especially a hen, because in my experience the roo's cluck the whole time) instead of a bantam, if thats what you bought.
Bantams are great, and my whole 4h club used them; but I think a lot of people didn't know you could use a standard bird for showmanship. I remember getting a few blank stares when I told people the bird I was using - "what your using a big one? Won't your arms get tired?" I guess my arms didn't, because I got 1st in a class of ten people with my big red hen.
I'm saying whether your Hamburgs are bantam or standard, you should be fine. Unless your son is young and can't hold a standard due to the hold your required to do because his hands won't be big enough; then a bantam might be in order, but as long as his hands are big enough to hold the standard; weight shouldn't be a problem b/c Hamburgs are pretty light.
