Bassman, I think Pluggy has the right idea.
Since you're not his dad or mom, you have limited information to write about him.
If he has an existing resume, I'd want a copy. If he can supplement it with notes for you to use (to flesh it out, and bring it up to date) he should do so.
Written is better than verbal... for you to write an accurate letter for him (
not on your memory of verbal conversations).
You are a relative, so it is expected you will be extra favorable.
Hit the main points: education, grades, leadership (in anything), no drugs or alcohol, no criminal record, no bad driving record, excellent work ethic (school and jobs), excellent physical fitness (vision, hearing).
Don't write a book, but be briefly thorough with best foot forward. And don't go off on tangents. One page is great, page and half to 3/4 max. Your letter must be perfectly grammatical. The letter should be a series of short paragraphs 3-5 sentences each (by subtopic), not long run on paragraphs.
When I applied to the USAF for a direct appointment commission (lawyer - judge advocate, at age 31, as old as you could be without special waiver) I got passed over the first time (they took like 18 of 60 applicants), and they asked me to apply again. And I did, and the 2d time I asked my congressman for a letter, and not knowing me from Adam, he wrote me a good one, and I was selected on that 2d go round. I found this pretty weird, but I couldn't argue with success. (My resume even without the congressman was pretty darn good; senior patrol leader of scout troop though sports success, and high college honors, student government, resident advisor, and more)
The funny thing was, I was a staunch conservative even then, and my congressman was a liberal democrat (but also a life member of the NRA). I knew him, but he didn't know me. Look up John Dingell (who holds the record as the longest-serving member of Congress in American history)

He got elected 30 times, and writing letters for people helped.