What we like for our archery education class for the younger kids is a bow that is 6-9# at maybe 20-24", but is at least 54" long, and is right or left handed. That way it is the most versatile. Your son will likely grow and strengthen quickly.....a longer bow will "grow" with him. Whereas a 48" bow that may seem just right for him now will be too small for him in just months!(sounds like you know that already.
These are hard to find. If you don't have a way to advertise or look for youth bows, many custom bowyers will make what you want. It's not inexpensive, but there will always be a market for it when you want to pass it on. Also, check out sponsors here and Martin Archery.