The growth plate thing is a BS myth....weightlifting actually promotes bone density, a good thing for teenagers. As far as age, the only thing holding a kid back is going to be emotional maturity.....some 13 year olds are more than capable of being serious in the weightroom and following direction, other 13 year olds want to play with GI Joe still....it all depends on the kid's individual maturity.
I started training when I was 13, 8th grade to be exact. The same rules apply to a 13 year old, or a 10 year old for that matter as apply to a 30 year old. I'd say the focus should be on a balance of compound lifts, learning proper form from day 1 and pushing for gradual increases over time, same as anyone.....don't push him faster than anyone, don't hold him back more than anyone, good, solid training plain and simple.
For diet, instill a sense of nutrition and what is good, useful food and what is absolute shit.....but don't forget he is kid. Don't expect him to run around with tupperware containers filled with brown rice and broccoli, that will backfire and you'll end up with the next Todd Marinovich, lol. Teach him to eat an abundance of nutritiously dense foods, and let him decide how clean/unclean he wants to be with the extras. If he wants to go for ice cream with his GF, then by all means he SHOULD, but only after he eats a double serving of meatloaf and mashed potatoes. Explain how nutrition relates to performance, and explain the difference between 'looking good' and performing, and let him go where he wants to go.
For me, when I was his age, it was a gallon of milk a day.....eggs, oatmeal, and toast with peanut butter for breakfast.....two or three school lunches with 4 milks (the REAL lunch, not the snack bar or the 'alternate' lunch of cheese fries and coca-cola)...two PB and J's before the weightroom.....then after lifting or practice I went home and had whatever my mom made for dinner, and lots of it with milk.....then did my homework while sipping milk.......then more PB and J's with milk before bed. Lots of veggies and salads with lunch and dinner.
Ih he wants to get big and strong, this is the perfect time because for the next 5 years his body will be naturally producing enough testosterone to make any juice head jealous.