The question I would ask is: Is your son ready to shoot at 40 yards?
I set up a shotgun (youth 870) for my daughter-she is 21 but has very limited trigger time-and put a Trulock HW choke and Fed HW#7. It is awesome at 40 yards, I will not hesitate to hunt with it. But I felt like 40 yards is too far for her to shoot. So I put a Remington Full choke in it and will limit her shots to 30 and in. It is pure poison at 30 yards, but still a wide enough shot to be forgiving. I snuck a HW#7 in to it yesterday when we were shooting (been using light loads) and she did notice the recoil a bit. But I had her shoot a turkey target and she was a little off to the right, but there were plenty of kill shots on the target for it to be a dead bird.
personally, I agree with gopher and would try the Fed HW#7 in 2 3/4 and a factory full and see what it looks like. Lead 6's will smoke one at 30, so you may be fine there too.