A kisser button can be a good start. But what I have found in the past it is practice, practice, practice. But, make sure you or the father of this boy are helping him with repetition. Ensuring he is pulling back to the same point. If he is not using a kisser button to help with an anchor point, is he using a knuckle against a cheekbone or thumb to a spot on the face, etc. Whatever it is make sure he does it the same each and every time. Then make sure he is holding the bow the same way, and his release (if he is using one) is fastened the same way and he. Look at things like that and then everytime he shoots watch his form and help critique him and before he shoots give his form a good look over and go through a mental checklist (but announce it to him what you are checking and if it is good or not). That way he learns what you are looking for and after a few dozen times or maybe hundred times he will start to do all this on his own.
Also, try to have him "blank" shoot to get the form down. Don't have him aiming at a specific target at 20 yds or 10 yds or whatever distance you choose. Instead, if you can get something large to shoot at (around me I can get to a farmer's house with round hay bales) and then let the person shoot into that. At this point you don't care where he the arrow is hitting at, but rather you are trying to develop form and muscle memory. This way the boy will be focusing on form and shooting rather then aiming.
I have found with young kids it takes them a bit to understand things such as form. Just like trying to teach a kid to bat or throw a ball properly. If you can break it into small steps or pieces I find they learn it quicker.