Well, personally, ive broken alot of bones in my body.
I also happen to be a nurse, and i work n trauma-orthopedics in my hospital, so i know quite a bit about bones
While i cant tell you anything for sure concerning your son'S arm. i can tell you that a residual bump can take years to disapear.
When the bone heals, at the site of the break, it created some new fibers, wich in time harden, and become new bone.
But it always created these fibers AROUND the break, making, as you could say, a *ball* of bone around the break.
With time, this *ball* of bone, will gradually disolve any unecessairy parts, to finish closely to what the original bone looked llike.
I broke my right collarbone like 3 years ago, and theres still a bump on it, though its less that wha it was before.
I broke my left fibula in august, and theres also a bump i can feel where the break healed... and mine is still a bit sensitve when i hit right on the bump. i guess that takes time too, and its different for everyone..
i hope this helps!