When my son was 11 y/o I bought him a Savage bolt action 308 Win; the package came w/ a Simmons 3-9X40 scope. He used that scope until I replaced it w/ a Leopold Rifleman 3-9X scope. The scopes already mentioned are all nice scopes, but you may want to considr starting w/ a lower end scope, then upgrading later as he matures. In general, kids are usually harder on equipment than are adults. We kept his shots to 100 yards at first, then lengthened them as he became more experienced. There's a bunch of ways of doing this, but how we did it worked out for us. Good luck. And most importantly, be sure you BOTH have a enjoyable hunts. That usually means that the adult will end up making some sacrifices!
ETA: Personally, I don't see the need for a 50mm objective on most deer hunting rifles; it adds weight w/o significanlty impacting the optics. I would NOT go above 9X on the upper end for an adjustable scope. Be sure he keeps the magnification on the lower end, for the most part, during the hunt.
This post was edited on 12/5 at 12:33 pm