Sure I remember the Whistler. I thought they were actually 13 feet. The are nice little half decked sloops. They were intended to compete with the Oday Widgeon of that era. I looked at one for my first boat when I was 12 or 13 or so. (I ended up buying a 10 foot Oday Sprite because my part time job would not support the bigger Whistler or Widgeon and they were hard to car top.) I was a pretty small 12 or 13 year old (less than 5 feet tall and under 80 lbs if I remember right), but my recollection is that the Whistler might be a little bit too big for a 12 year old to single-hand. (even my 10 foot Sprite was a bit much) I do think this is a boat that your nephew could grow into and really enjoy. I beleive these boats went out of production in the 1960''s (somewhere around 1966) and so this is an older boat and should be looked over quite carefully.
As an alternate, you might look at a 420. 420''s are self rescuing meaning they have a better floatation arrangement and can be righted and sailed away again. The Whistler will be swamped and will not be able to sail back on her own. Which brings up another point. Many of these older designs lacked proper floatation or had strangly distributed floatation. You should probably check this before you let your nephew out on the water.
Good luck with this (Your nephew is lucky to have an Uncle like you.)