Throwing and Running...

I've seen some good ballplayers go through a season, following a growth spurt, where they were as awkward as can be. Work on getting back to basic throwing mechanics.

Hamstring problems also often accompany rapid growth.

Stretch and agility drills will work wonders. Has he moved from a Little League sized field to a high school sized field? That could contribute to the weaker arm - where I am 13 is the first chance a kid has to move from the little field to the big field. Not sure if that is the case with you.

Well... in part the size is a factor, but not a dominant one. As i mentioned before he "forgot" how throw a ball. It looks really bad, I guess with the growth went his "muscle memory".

Maybe you can help him try to relearn how to throw? Draw a black stripe w/ a Sharpie around a baseball, right over the 2 seams that your fingers would lay over on a 4-seam fastball grip. Play catch w/ him, having him align that line between his 2 fingers on a 4-seam grip. Watch for the black line not to wobble. If he's coming off the ball sideways or has a "funky" arm angle, have him keep trying different releases or arm angles until he can stop the wobbling.

No guarantees, but it's been helpful w/ my players.

The other thing you might try is having him throw in the living room into a pillow on the couch, from one knee about 8-10' away. My son did this for several weeks this winter (as part of rehab for an elbow injury last year) and it helped him "remember" how to throw like he did before the injury last April. So far... fingers crossed, as he's throwing/pitching well with no pain.

Good luck.

As players grow, yes, you have to continually reteach them.

Beware of the "Stork Factor". Growth spurt where the kid doesn't use his core at all and looks like he's leaning over all the time. Continually retrain and work the core muscle groups.