Increasing pitch velocity for soon to be high school player

My son is a freshman and 14 yo. He is currently 5'10 175 lb but was around your bro's size last year. His velo numbers is above average. My son has been following Driveline Hacking the kinetic chain youth edition for a few years. Last year he started a strength and conditioning program. His velo increased from 70s to 80s this year.

Don't worry about velocity right now. Making varsity as a freshman is nice, but it's not everything. You're from Maryland, most HS pitching will be closer to 75 than it will be to 90 anyway so assuming he continues to grow and gain velocity naturally he won't be too far off the average varsity pitcher. Just throw strikes and he'll be alright. 

The thing that is preventing him from higher velocity right now is mostly his size. 135 lbs is very light. Just have him eat, try to get that weight up and the height should follow along with it. I wouldn't start weight training just yet, give it the summer and start in the fall. People can dispute that, but rapid growth + lifting can cause some injury problems, especially if nobody is supervising him closely. If he wants to get stronger have him do pushups, sit-ups, pull-ups and unweighted squats/lunges for now. See where he's at in August and go from there. 

Throw long toss. It is not too early to build a strong arm. And syrong arms are not just for pitchers. Throw Jaeger style long toss several times a week.