Gotta get some "man muscle" on the kid's body. Sounds a lot like my son. He was (and is) all fast twitch, thin as a reed and as fast as the wind. He was an outfielder in high school and college (briefly).
He could chase the ball down with the best of them. High school track coach, who helped him improve his 60 times when he decided he was serious about playing in college, actually tried to recruit him for his summer elite track club ... said he had the makings of an elite sprinter, just needed to work on his technique.
He could also hit. (well he was a tad bit streaky. he had his can't miss it games and his can't hit it games.) But he did have a really sweet swing with enough bat speed to handle just about any fast ball he saw in HS, travel ball, and at the D3 level. As the pitching got better, it was wicked sliders and curve balls that were his downfall. But even if he got hold of a fastball, which he often did, it wasn't likely to go out of many parks -- though he did hit one granny once and was a doubles machine, when he was on.
In HS, he got get a few sideways D1 looks, mainly because of his foot speed and his bat speed but every single one of the D1 guys who looked at him basically gave him the message that he needed to put some serious man muscle on, if he hoped to seriously compete at their level.
His HS strength and conditioning coach had the goal of getting him to 170 by HS graduation. " If we can get you to 170, you will be unstoppable" he used to say to the kid to try and motive him.
Unfortunately, that never happened. I think that was partly that because the kid just wouldn't eat like an athlete. Ate like just your average American teenager. Not good for building man muscle.
He did manage to touch like 145 his freshman year in college, but he's back down to 135 after college. Rapid fire metabolism, not a lot of time to cook, and a tight budget, I guess. He played well enough at his elite D3 his frosh year, but for various reasons decided to voluntarily hang 'em up, sophomore year. Biggest surprise ever. Didn't see it coming.
Anyway, bottom line fast twitch small guys who are super athletic have a pretty steep hill to climb. There are a fair number of smallish guys at some good D3. But "man muscle" matters even at that level.