martin makes some good bows that have very large weight variences... My girlfriend shoots one thats 35-50, which is good cause she can deer hunt w/ it. But they make lighter ones... Id suggest starting him round 20-25 lbs,keep it light so he can work on his form and learn the art of archery.... if you start out struggling to pull to much weight you'll develop poor form and that can take years to overcome...

But overall martin makes good bows, also browning would be my 2nd choice, they make some awesome youth bows that have the right draw weights

You might want to take him to a shop and get his draw length measured, and make sure you get a bow that you can easily adjust the draw length so it can grow w/ him.