Hey! I'm new to having a tank myself, but I work at a pet store, so one thing we tell people to keep a "full" looking tank is to get fish who stay in different areas.
For example, cory catfish are small, active fish that stay at the bottom of the tank. They love living in groups, so two or three will put on the best "show" for your son.
Danios and tetras come in many colors, and are active middle swimmers. They like to school as well. They can become aggressive or shy if kept in groups smaller than three, but depending on the type you get, having four or five should not be a problem.
Guppies come in MANY colors and don't have to school. They stay very small, so you can keep a small army
They are prolific breeders, and the males are most colorful, so keeping males-only is preferred. They are separated at the pet stores.
White clouds and Hatchetfish stay to the top of the aquarium. Both are schooling.
One word of advice I'm telling people at the store: Unfortunately , the dwarf gouramis (which are BEAUTIFUL fish) have a virus going around....it's VERY widespread and coming from the breeders. It's untreatable and fatal, but it does not spread to other fish. I'd stay away from them for now though....hate to have your son get a dud if it can be prevented.
Good luck!