I think that the most important requirement for a first bow for your son is that it doesn't require him to struggle to draw it. The same can be said for you as well. If it's too difficult for your son to draw the bow, he won't find it much fun and will soon lose interest. If you get a bow that's too heavy in weight, you won't be able to develope proper and consistent form, which is critical to consistent and acceptable accuracy.
Have fun with it. It's a wonderful sport that can teach much more in life, than how to cast an arrow.