Go to a pro shop and shoot several bows, new or used and find one that fits him.
Having a bow with the right draw length and one that feels good to the shooter is important.
Nobody can tell you which is best for your son.
There are lots of very good compounds these days.
This, it's well worth getting a bow setup for him.
With a bow setup for him, he can put his effort into learning to shoot the bow accurately. With a generic used bow that isn't setup for him, he'll have to learn to tune a bow and learn to shoot it at the same time. Which can easily lead to a lot of frustration and not knowing if accuracy problems are due to him, or the bow being setup wrong or using the wrong arrows or...
