I have a nephew that got into weight lifting / bodybuilding around the same age (not for sport). He's 17 now and is very serious about it, has made great gains in the last 2-3 years.
Given that he's a kid, has other things going on in life, etc. - I just gave the friendly suggestion of hitting his weight in protein grams per day, and then whatever else he might want on top of that.
It's just one number to pay attention to daily (so if he's 150 pounds, get in 150 grams protein daily) but it teaches him how to start paying attention to stuff like that, and if he's truly interested in it and sticks with it, just hitting that number daily will make him *probably* hit the calorie requirements he needs to grow. At 14 he's basically a freshman in high school, so TONS of other things going on in his life - school, friends, learning about life in general.
Tangent, but cool: my niece's boyfriend is exceptional at basketball, just finished one year of college basketball and got signed to the NBA (Charlotte Hornets).