I would find out more about exactly why he wants it – is it for college? What college? Is he applying for a scholarship? I work at a college (and review applications), and what I want to know is not only why is this kid going be great at college, but why is he going to be great at MY college.
Obviously, for academic potential we are usually looking at rec letters from teachers, so if you are not his teacher, I don’t expect the letter to go too much into that.
For a letter from someone not a teacher (scout leader, athletic coach, youth group adviser, employer, etc), I would be looking for specific examples (Voyager has good advice) the illustrate qualities such as:
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Works well independently and is also a good team worker in collaborative situations; takes responsibility to fulfill his commitments. Shares credit. Can manage his time and plan assignments. Can break down a larger objective into specific tasks. Leads by example.
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Persistent, resilient, can learn from mistakes or set-backs, sees them as opportunities to improve, encourages peers to do the same (even better if you can show examples where an initial set-back led to later success). Rises to meet challenges. Sets goals that are realistic, yet require him to push himself harder.
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Open to understanding different viewpoints and learning from others; seeks opportunities to engage with different kinds of people; is thoughtful and reflective about new experiences.
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Intellectual curiosity outside the classroom, pursues interests independently, learns for the sake of learning.
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Contributor to his community (school, local, whatever); willing to give time and energy to worthwhile projects. On this one, I would always rather see a kid who gets involved with one particular cause, issue, or volunteer initiative in a sustained, ongoing way than a kid who volunteers briefly at a million different things.
Depending on how you know him, you probably can only speak to a few things, don’t try to stretch to include things that you really can’t support with direct examples (because the people reading the application can tell).
Things like polite, respectful, and being generally nice are okay to mention once, but unless that somehow relates more deeply to a specific quality you are trying to convey, we pretty much assume that anyone writing a letter of recommendation thinks the kid is nice.
If you know him as a family friend, don’t spend too much time on this - usually when I read letters from a friend of the family who goes on and on about how long they have known the family, and how great the parents are, I am getting the impression that the kid didn’t have anyone else to ask who could speak to his abilities and potential. Get to the part where you explain the specific role in which you can evaluate him as soon as possible.
A full page letter is generally the most helpful. If it goes much longer, I start skimming unless there is something completely compelling going on. If it’s shorter, I don’t have enough information to go on.