First thing I notice is that he stays straight up when he throws, his velocity comes from all arm power. You might notice at about :07 sec. that his body never gets any farther down than that, he needs to work on pulling the ball down, ending with his right wrist just above the left knee, this will help to get his back flatter to the ground, there should also be more action of his right leg coming over to help decelerate the arm action, this will happen naturally at first from him getting flatter.
Second thing I notice once I watch all the video’s is that his timing isn’t always the same as he starts to come to his post, once he starts to come to the post all the mechanics should be the same including timing.
Did I hear you say to him, “No cutters, cutters are hard on your arm”, and then turn right around and ask him, “How about a curve ball”. Well honestly I think you son at 12 is too young for both those pitches but you are the best to decide. The cutter is truely just a 2 seam fastball with the fingers set to the right a bit and the wrist pre cocked, but the arm swing is the same as a fastball so it really is no more stress on the arm than a fastball.