I remember back when I was 10-11-12 years old my first ball my Dad bought me was the Brunswick Heat...10 lbs. conventional grip. I didn't get a fingertip ball until a few years later, 13-14 years old. That was a 12lb. Brunswick Red Heat.
So for your nephew I would suggest something like a Tropical Storm, Power Groove, Tornado, something along those lines. To me I wouldn't want to spend too much money, especially when at 11 you grow like a weed. In a few years if he keeps pursuing the game get him into some mid-entry higher hook potential balls.
This is the time to develop the raw talent and fundamentals.