Last year one of my grandsons started his first year of highschool. He has always made good grades, but for some reason his science grade dropped considerably and some of his other grades were off some. My daughter cut back his hunting with me, restrictions also. Turned out his eyesight required glasses and the science teacher was not a very good teacher and a lot of students had a problem with her. I don't think stopping all his hunting is the right thing to do. You need to remember that time with his grandfather is extremely important to his grandfather as well as your son. Lots of life's lessons can be learned with their time together.
Has he been evaluated for ADD or some other potential problems by a professional? I don't know the cure for the problem, but it doesn't seem that the route you are taking is working.