I thought the same thing, and let me son spend his money from his birthday last year (when he turned 7) on a fish tank and fish etc... But guess who ended up with the whole cleaning the tank process and maintaing the water ph levels and all that other junk! Depending what size tank you get, it's a chore to maintain it all! We ended up giving the fish to a friend, and adopting a Guinea Pig (from either petco or Petsmart, you can adopt unwanted/returned small pets such as hamsters, guinea pigs etc for free or you can donate if you want, we donated a lil money) anyhow, way easier to take care of then fish! We recycle our newspaper by shredding it up for him, and my 2.5y/o helps my now 8y/o with the feeding (supervised of course). You might try a small hamster or gerbil. You can buy a ball he can watch him roll around in, and a wheel... More fun to watch, and a tad easier then fish almost! Good Luck!
P.S. We had a Beta, Danios, gupies (those lil suckers breed like crazy, avoid those!LOL), a snail, algae eater, there was a shark type fish.... Anyhow we tried all of those, and of course lost some a long the way... Some fish just don't live to long! Be careful where you buy them, make sure there aren't a bunch of dead fish there etc...