GBM, I will not question whether or not what you are doing for and with your son is good or bad or whether it is best for him or not. I simply don't know your son or your situation. I will say that you speak with a lot of confidence that I don't think is well founded yet. Your son is only 14, so, quite frankly, you really don't know whether it will work in the long run or not. Yet you still talk like what you have done with him is what should be done.
Also, around here, mid 70's for a 14 year old is fairly common. Summer before my son's freshman year, he was throwing 82. Another kid at 13 and 14 was considered a hard thrower and threw about as hard as my son. At the same showcase between freshman and sophomore years, my son threw 86 and this kid threw 74. Everyone thought that kid had great mechanics. You really just don't ever know.
Another personal story. My son was like yours in that through 14 years old, he never got sore, seemingly never got tired. Once he turned 15 and started throwing in the mid 80's, he did start getting sore and had some minor elbow issues. I believe that part of that was a result of his velocity and quite frankly, part of it was from throwing too many curveballs. We cut WAY back on the CB's this summer and he has been fine - Arm wise and effectiveness wise. All this has nothing to do with size either. He has always been small for his age. Currently, he is 5'9" about 170 lbs. Has hit as high as 87 with his fastball and usually cruises between 84 and 86. He is going into his junior year.
I also think you need to take into account the competition your son faces. Sounds like he is playing rec league ball. If my son were playing there, he would have some no hitters and a ton of shut outs. He plays the highest level of competition available.
Anyway, bottom line is, I really don't think your son is old enough or experienced enough to know for sure whether what you are doing is going to work or not. Just don't think you should be so adamant about your theories until they can either be proven or disproven over time. There are a lot of people here who have seen much more than you. There are plenty of people here whose son's have made it to a much higher level than yours. You should really take into consideration what they say.