As far as which bow to get him, you will need a bow that he can grow with and his adjustable and can be changed to accommodate his ever changing young body�visit multiple shops and have him shoot a wide range of bows for what he is the most comfortable with. All of the bows previuosly mentioned are great choices, and I may add that the Mission bows have a great selection of youth bows that you may want to consider as well.
To obtain his draw length here are a couple of ways you can measure that will get you close�although, you should have him measured by someone that is very experienced and can get him measured correctly because that is the building block of everything that he will use in archery. Number one have him stand erect, but do not stretch, with the arms straight out to the side, forming a "T", and the thumbs up and measure from the tip of one middle finger to the other and then divide by 2.5. Another rule-of-thumb measurement is to have him stand in front of a wall in a stance like you are going to shoot a bow and make a fist and then put the fist against the wall directly in front of him at nose level and measure from the wall to the opposite corner of your mouth. Keep in mind that both of these are only guidelines and will get you in the ballpark.