"CADAD just gave you some sound advice and we have been long tossing all winter long."
This is troubling advice. Everyone, including major leaguers, needs to take time off to allow their muscles, tendons, and ligaments to heal. That is even more true for kids under 16.
"As far as the weighted ball program goes, if you have sound mechanics it shouldnt be a problem as long as you monitor each and every throw. Believe it or not a weighted ball will also highlight the bad mechanics."
Are you suggesting throwing with standard mechanics using weighted balls? If so, then I would be very careful. Weighted balls can be a useful tool but with caveats. First, they should only be used by kids who are older than 16. Second, they should only be used in drills (to strengthen muscle groups). Trying to throw a weighted ball 70, 80, or 90 MPH is a recipe for disaster.
"Also, a changeup while in Little League will probably be sent over the outfield wall. Definitely start to develop one but when you are throwing at Little League distances it will not fool many people. IF you have a very good fastball then the changeup will be effective."
By definition, a change-up isn't a change-up if there is no change. An effective change-up must be thrown in exactly the same manner as the fastball but with more skin on the ball so that it will come in 10% slower.
"Currently the son is 13 years old and has good mechanics. No arm or elbow problems ever. We started a throwing program after his last Tourney in early December. Velocity has gone up about 4-5 MPH in a span of about 2 months of hard work."
Be careful. He is just entering puberty, which is the real danger area because the strength of the muscles starts to far outstrip the strength of the growth plates of the bones.