Tom,
It looks like a really great gun package for the boy. Ur...Uhhh...Well don't forget it really is for your nephew.
All kidding aside, you may actually want to test it for him. A 5# shotgun, even in 20 ga. might have more recoil than is desirable for a younger boy. (Of course I am assuming you are not from one of those interesting families where the nephews are older than their uncles!)
If the recoil seems a little heavy, options include: Installing weights in the stock; installing a mercury recoil reducer (I assume they work); installing a Pachmayr Decelerator recoil pad (they're the best I have used); and installing an gas, hydraulic, or spring shock absorber. The absolute worst case analysis is you have new gun project to work out for him. Your fertile mind probably has more and better thoughts than I have come up with.
Whatever you do, please keep us apprised of how everything works.
Good luck,
John