I can't think of any reason why his age would really make much of a difference. When I've had people ask me about this (I'm a pharmacist) I've always told them get a few different brands and flavors and to make the decision based on palatability.
To name a few that are basically everywhere: Muscle Milk, Myoplex, Ensure, Glucerna, etc...
The most important thing is get the calories/nutrition in. I'm assuming you want pre-made just pop the top and drink.
Milk and chocolate milk may be option depending on circumstance.
What "junk" are you worried about the Ensure?