I love leghorns, but they can be a bit flighty. White cochins, orpingtons, or rocks might be a bit more calm.
we have a brown leghorn and it is our only chicken that won't allow us to pick her up. so if this is true for all colors of leghorns I would consider something else. She isn't mean, just skittish. we have one silkie (that is black) and it is AMAZING for kids. They can be white too. She lets anyone pick her up. She is quiet and sweet. She loves to be held. If the silkie is raised with the others from the beginning it shouldn't be a problem. Ours is doing great. We have 12 total. (barred rocks, orpington, marans, silkie, brown leghorn, black sexlink, and easter eggers. My only occasional peckers are my barred rocks. Good luck! It is SO fun. My kids love it.
The problem with silkies is, it is nearly impossible to sex them early. Generally 4-5 mos. is the earliest you can tell. Sometimes not till it lays eggs or crows. If sex doesn't matter - just pick one your son likes., or actually two. Chickens are flock animals and prefer to have buddies. If your son wants a silkie as a house pet, your family would be her/his flock. Please check out "people with house chickens," thread.