Ask your son to explain what the pitching surface is like that he has to work off of. These amateur ball fields can be a struggle for youngster, not to mention destroying any form and learning curve of how to pitch.
I totally agree with fearsomefour on playing catch. A simple game of catch can not only develop total body movements that compliment the pitching process, in addition to impressing a "rhythm" for your son to use with pitching. All in all I wouldn't be too concerned with the firepower aspect right now. If your son can work the strike zone now, he's light years ahead of his contemporaries. Also, by working the zone, he probably gets the rest of his crew on the field involved in the game as well - that's a good thing.
This velocity thing for any youngster under 18 requires a broader perspective that what's usually suggested by those on the sidelines, not involved with the total game and so forth. Building a body and mindset that will accommodate the disciples of sleep management, diet and nutrition training tables, conditioning for the age group and personal demeanor of the youngster, is a limited but accurate way to chase the velocity/game control golden goose. Of all the topics that I just mentioned, diet and nutrition is key to all other doors that you want your son to open. The values of carbs, proteins, vitamins and mineral impresses to health growth and development are mandatory foundation blocks for everything that follows.
Oh the other hand, I'd say a large percentage of youngsters and their fathers usually go the .." I want 90+" first on my pitching resume, with the foundation stuff far behind.