You would want to work more than just the arm. Pitching is a movement that makes use of the whole body, so a drop in velocity can be caused by any portion of the chain. There are so many different options as far as working out. Check out some of the threads in this subforum to get ideas. I would start out with bodyweight/ resistance bands for a couple weeks, then go with some freeweight movements (start slow and build up).
The other thing you should be looking at is teaching your son about nutrition so he can add weight to his frame. 6'1 and 140 is really skinny, and that could possibly be holding him back as far as velocity and consistency. Again, check the other threads in here to get an idea of what type of diet you should look into. There are a couple guys that are about the same weight as your son, so the program would be similar.
But, overall I would just encourage you to start slow and stay consistent. A lot of parents and kids go gung ho on stuff like this for 2 or 3 weeks, then they get bored or busy and forget about it. Gained muscle can be lost in a matter of months without continued work. It's a lifestyle change you have to adopt.