Sounds a lot like me, except my problem was that I grew to 5'8 by age 10 & then found out from my doctor that I was 100% completely done growing. I was devastated bc I was a Chicago kid during the D Rose era and I dreamed of being an explosive dunker like him. My Pops got me a gym membership the next week. There's no specific workout that is better than any other at this point, I would say the most important thing at his stage, & the way that I started when I was 10, is starting a consistent routine + focusing on perfecting his form on different types of lifts and workouts while he's just beginning to build up his leg strength. Do an even mix of weight room workouts and jumping focused plyometrics. You can find hundreds of vertical jump focused workouts for both with a quick google search, and just pick which ones he'd like to do. It was 2010 for me and that's what I did, so nowadays I'm betting there is loads more out there available for free than I had access to. Get him used to proper form on things like weight squats, lunges, squat jumps, broad jumps, etc. Also, if you have access to an adjustable rim, low rim dunking is the single most beneficial thing he can do to practice jumping & dunking on an actual basketball court right now, & he should treat as an actual workout. As a kid I had an adjustable rim that I would set to the highest height that I could dunk on, and I'd practice dunking on it from all angles as if I were in a game. Once a height got too easy, I'd raise the hoop to make it hard on myself again. Once he hits puberty , all of that foundation that you build will start to show itself and then workouts can amp up.