Welcome to the Tank,
lisa1976!
I think you're likely to get suggestions that you get a betta for him (maybe not here, but probably at a pet store). And, that might not be a bad idea. But, don't be misled into thinking that that betta will require less care than any other fish. They can get by in a smaller tank (I think anything less than a 5-gallon tank is a little cruel), but they still need warm water and oxygen and the like. But, bettas can be a little on the easygoing side; a betta might not be as fun for your son to watch.
If you go with anything else, I'd recommend nothing smaller than a 10-gallon tank. And, make sure that you don't overstock it. The mollies and platies and guppies that
brian1973 suggested aren't a bad idea.
What I would basically suggest is that you figure out how much support and care
you are willing to take to maintain the tank, as you realize that you'll be the one to have to do it. Once you've done that, I'd look through all of the stickies on this forum to get some of the basic information. Use the search feature, too. And, don't be afraid to post with follow-up questions; the sticky information can be overwhelming.
One more thing: watch out -- this hobby can be really, really addictive!