Throwing a heavy ball....

Just an opinion - I'm not sure that even the experts can tell you how to go about throwing a heavy ball.  Some pitchers just have it.  Guys at the higher levels who are associated with throwing a heavy ball have sinking action to their fastball.  Even though they may be throwing 90 mph, their ball sinks (and it actually does feel a bit heavy when you make contact).  This results in a lot of ground ball outs.  In my observations, I think most of these guys tend to be slightly out toward 3/4 with their arm angle.  So, if there is anything to what they are saying in regards to your son, it may be tied to keeping his current arm angle as he develops his velocity.

I'm really not sure though and curious to hear others thoughts on this one as well.

New to the forum, but definitely like the quality of discussions.

Most of my experience with pitchers who throw a "heavy ball" are as stated before, a FB that sinks, typically two seamer with some run, and sink to it...tough to always square up in the mitt when catching, or on the bat when hitting, because of the run/sink, hence "heavy ball".