If you don't feel comfortable enough, leaving a handwritten note explaining some of the things might suffice. As a man, I can say that the worst part of being a teenage boy is that once you discover masturbation, you'll be doing it a lot. Suddenly you're thinking about sex so much, and getting frustrated that you're not having any sex, etc. It really sucks. Other than that, boys' body changes don't feel special or different or anything. Extra hair grows here and there, voice might finally sound masculine, and that's about it.
Some things he should know, from someone who experienced it first hand:
- Shaving pubes is not necessary. No need to bother with it until you start getting frisky with another person - and they might not even care whether there's a bush there or not.
- Try to pull the skin down as much as comfortably possible when masturbating. This is important for having sex in the future
- Clean up the head with water and soap, then dry it before putting it back inside the clothes. It's also a good thing to use toilet paper to dry up after taking a piss.
- Water + semen = glue. You need lots of soap if you don't dry clean it first.
- Keep some toilet paper, towelettes or an old piece of cloth nearby for the cleanup. Wash the cloth at least once a week.
- Install uBlock Origin. He'll be browsing porn sites whether you like it or not, better have some protection against the nastier ads, which tends to be really invasive.
- Masturbation feels good, but don't overdo it, unless he wants to have erection problems when he finally has sex.
- Adding to the above, it needs to be driven into his skull to NOT expect anything he sees in porn to be realistic. Positions, dirty talk, all of that is for entertainment, not a how-to.
- Keep an eye on your credit card, OP, or you might suddenly see a surprise charge for something very unexpected