Same thing happened to my son. His joints got stiff and he was running flat foot - or something. He is now about 6'5" - only growing 2 inches this year.
He is working with a personal trainer and is looking so much more coordinated. He is working on getting his swing back too. He is upset that he is having to relearn. (He went from hitting line drives in his sleep to chopping at the ball. Long arms mean a compact swing isn't what it used to be) His pitching has stayed strong. He has added velo and his curve and off speed are nasty. He had to be mindful of his release point a bit - but putting that weight and angle to work is pretty.