My son is 14 and entering high school in the fall. I started teaching him to pitch at 8. While he pitched very well this past 2016 season (his best season ever), in hindsight, there was no need really to start him so young, 8. Mariano Rivera started pitching at 20!
I would focus your son on proper throwing mechanics and proper batting mechanics, and fielding. I would start teaching him to pitch at perhaps 10, and always follow very carefully the MLB Pitch Smart guidelines for youth pitchers, click here, as he grows and matures.
Importantly, protect your son. Do not listen to all the “advice” you will hear about the need to play baseball year round, to pitch in multiple games on weekends, etc. No one out there is going to protect your son’s arm; it’s going to be your job. I myself am getting ready for potential conflict with my son’s high school coach next year, as my state has only a ridiculous “14 innings per week” pitch limit, which basically means my son can pitch two complete 7-inning games each week regardless of pitches thrown or days rest! The coach is old school and I’m not sure how he’s going to take to the MLB Pitch Smart guidelines I want my son to follow.