throwing speed

Good advice from everyone so far. One more important piece of advice. If your son does get called upon to pitch, educate yourself about pitching safety. Stretching, proper mechanics, pitch counts, stages of pitching.

For pitch counts, the first thing is to just be aware of is exactly how many pitches are thrown. More and more teams are keeping track, but still not very many do at the beginning levels. Don't just rely on the league rules to keep a pitcher from pitching too much. It's the number of pitches that are more important rather than the number of innings. And the rest between outings, especially if high counts are involved. High counts in a single inning can be just bad even though the overall count may be "under the limit". Much more to cover on the subject. Educate yourself from books and places on the internet like this one.

By stages I mean location refinement and type of pitch. For location, start w/ fastballs down the middle and progress to hitting desired locations as the pitcher's accuracy improves.

For type of pitch, start w/ 4-seam fastball. As mechanics and location become reasonably proficient, add the 2-seam fastball (exact same mechanics, just different orientation of the ball). Again, when location is reasonably mastered, add a change up (exact same mechanics - notice the theme here? - just different grip). The pitches mentioned above will reinforce basic mechanics (same for all) and develop strength. There is no need to consider any other type of pitches until these pitches and their location have been "mastered".

Note that the time frame to do the above is not a single season. Probably 2 full seasons minimum to get anywhere close to introducing the changeup (by the way, start with the stretch only for a minimum of 1 season before introducing the windup - 2 seasons would be even better in my opinion). The changeup will take at least a full season of development as well. Even then, it's best to take another season or so afterwards to work on mastering mixing the pitches effectively, learning how to work a batter based on count using the change of speed, change of location, and varying grips slightly (and even changing position the pitcher stands on the pitching rubber) to get a little movement.

Sorry this got so long and is a HUGE stretch from the original question about pitching speed. I hope it is of some use.