First, do research on your own and have him do research on his own. Compare what you find for how to set up an aquarium that would work well and be stable. If you haven't already, learn about treating water to remove chlorine, and the nitrogen cycle and why it's important.
Decide how you will decorate it before you think about fish, especially if you have no experience with tanks, since while it might be sad, chances are however you decorate the tank will probably outlive whatever goes in the tank. I suggest live plants. They can really help any tank, and the fact that they grow and change (some grow quite slowly though) adds more interest to the tank, and makes fish feel more comfortable. They aren't actually hard to grow, for the most part. They can actually become the stars of the tank, just look up pictures of planted tanks!
Most of all though, please don't go out buying fish just because "they look cool". Remember that these are live animals, and do have requirements. Research everything and anything you're thinking about putting in the tank. Sometimes you find out that something you thought would be fine, is in fact, not at all suitable. Take goldfish for instance. While most people think they do fine in bowls, in reality, each goldfish requires at least 20 gallons, and each other fish requires 10 gallons more!
Having a good tank takes time and patience, which is why they don't make great pets for children. I'm not saying that your boy will get bored or impatient and forget about the tank (it's the parents job to make sure pets get taken care of) but it is a lot of responsibility and hard work.
All that being said, decorate and cycle your tank. Once that is done, come back and ask about fish. It make take awhile, but I promise it can be worth the wait. (It's better than fish dying left and right!)
9yr old 20g tank. whats good for him to have
