If you're interested in finding exercises to work those muscles you can search http://exrx.net/. A great resource for anything related to working out. And free too. That's the best part.
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You click on the areas of the body that you want to find workouts for and it will show you stuff that you can do.
If you are having him do push-ups, then you can add other body weight exercises to work the upper body as well.
Pull ups and dips are two great exercises to work those areas of the body.
If you are doing the Jaeger band routine, the flies and reverse flies work the back well. You can also add in ' bent over reverse flies' with a light dumbbel(5-10lb).
For those you basically stand, have your back at around a 45 degree angle and do reverse flies, focusing on the pinch of the scapula.
An easy way to make sure that your son is strengthening all the muscles he uses to pitch is to develop some sort of full body workout. I don't have any specific formula for making a workout, but whenever I do, I focus on the major muscle joints: chest, shoulders, legs(quad, ham, hips, calf), abs, arms(bi&tri).
If he is 14 then he can probably start lifting weights and heavier ones at that, but I would focus on form rather than weight. At his age, he shouldn't be going crazy trying to lift tons of weight, but could benefit from a full body lifting routine. If focuses on form, he will remain healthy and be able to build up some strength.
Like my dad has always told me(phys ed teacher and trainer) you workout to prevent injury. Strength and muscles are side effects of that.