The first step is to find a good archery shop to deal with.

Go in and explain exactly what you need for your son. Pay for a few lessons for yourself and him if they can offer that with shop bows. Then buy a bow for BOTH of you. I doubt he will stick with it if he doesn' t have a partner to shoot with. My 7 y.o. is now at the point that he shoots more than I do. He was out this morning in a POURING rain and I couldn' t be convinced to go out. This is the greatest father/daughter/son sport around. Now if I could just get my wife to shoot.