I think this is the point where alot of people get frustrated and decide not to keep fish. Please do not do that! Fish keeping is one of the most rewarding hobbies I know of. At first it seems soooo complicated. Once you get the tank cycled, things will go so much better. And places like this didn't exist when I started( there wasn't an internet)

Just take a deep breath and start doing some research. First, learn about cycling your tank... then start cycling. Don't worry about what to put in the tank for now. Just get it cycled and start creating a living area for your creatures. It will all fall into place. Get your son into the cycling process. Have him help with the water tests. Explain to him that you have to get the water ready for the fish to live in.