TPM,
Every time we've had a chance to work on my son's velocity for an extended period of time he's added velocity. When he stops making the effort to throw hard on a somewhat regular basis and especially when he's throwing a lot of innings and can't safely throw pens it tends to get inconsistent. He's probably near his genetic limits at this point topping out at 88. He projects to top at about 91 according to multiple sources and we'd like him to get there sooner rather than later but for the moment we are working on his release point and cleaning up the mechanics. Once he gets a break from the college program and after a couple weeks off to rest his arm we'll work the velocity for a few weeks.
Yes, the weighted ball work did help him be a better young pitcher, but over the long term long toss and simply throwing hard pens and throwing hard in games when appropriate has done the most for his velocity.
Personally I'd just like to see a bit higher and more consistent working velocity and could care less what he tops out at. Normal increases with age would have him topping out at 90 to 91 by the time he graduates from college.
My son had an arm injury at 14yo from throwing too much from 3rd base due to an absolutely unqualified HS freshman coach. The injury was an olecranon stress reaction and was growth related. He inherited relatively porous bones from his mom and it is just something we had to keep an eye on while he was still growing. We shut him down and it was recovering just fine but the totally unqualified 18yo HS freshman coach started him at SS before he had recovered when I didn't show up to stop it. He won the position but reinjured the arm and lost an entire season. Fortunately, he's never had a ligament or tendon injury. He did suffer a small muscle tear in his brachialis muscle during physical therapy when they used the wrong setting on a Cybex machine. That's the extent of his arm injuries other than minor inflammation due to irritation of synovial plica in his forearm which probably resulted from the stress reaction at 14yo. Will he go the rest of his "career" without an arm injury? Who knows? He'll almost certainly have to get the plica scoped if he keeps on throwing long enough but we can only hope he gets to keep playing that long.
Now why don't you go ahead and give us some detail about your son's arm injury history.