There is a book about being shomer negia by Gila Manolson, called “The Magic Touch.” It is written with sensitivity and is not explicit, but is very educational in an extremely logical sense. It explains the practical benefits of being shomer, and the negative outcomes of breaking it.
IMHO it is teenager-friendly, and if this is something that this boy is struggling (or will be) with, then you should not worry about tiptoeing around… From what you said about his environment, I would assume he is not naive and you should be open and honest with him.
There are many, many people who read it and became shomer negia. It definitely helped me when I was younger… I would highly recommend it.
You can read it first yourself and use that as a guide for what to tell him, but I would suggest giving it to him to read himself. It is a very easy read; it’s thin and has no fluff.
Good luck, I would like to hear how it goes!