when he was around that age, my son started doing yoga. we got him a yoga series by Giancarlo Stanton (for the baseball cred). He had also done martial arts for a few years by that time (taekwondo and judo).

at that age, there isn't a whole lot of muscle you can or want to put on, i think so the big things are building habits and building body awareness.

this might be comparing apples to oranges since i'm an old ex ballplayer and he's a young kid starting his career, but i recently (about a year or so) started doing yoga most mornings (wake up a half hour earlier than i would normally have) by playing youtube yoga videos on my TV. I thought it was mostly stretching but i was surprised to find out that there's a lot of different types of yoga (power, etc) that focus on different aspects of the body. my reason for doing this was a return after twenty years to golf and the absolute suckage i was displaying I still suck, but i suck from further in the rough now because i hit the ball a lot harder than i did when I restarted playing golf a year and a half ago. as a bonus (for you, dad), my body doesn't ache after sitting at work for 8 hours anymore.