What a great calming solution for a child with autism. Pure genius. I don't know much about the condition but is there a possibility he might become too obsessed and it would slow the development of learning how to interact with people? Just a totally uneducated thought.
I would purchase a good basic book on freshwater fish and read about how to cycle your tanks. Educate yourself about the impact of water hardness, Ph, as well as suitability for fish with each other. If you're going to have a community tank then make sure ALL the fish are rated "peaceful" - I made that mistake once and now have to have a separate aquarium for my semi-aggressive fish to keep my peaceful ones stress free. It was a nightmare.
Websites that sell fish online often have a lot of great information about any fish they sell (and many sell far more varieties than a fish store) so even if you don't buy your fish online - read up. Also try Wikipedia as a source. I buy my fish from LiveAquaria.com and they have great info on the fish MOST of the time - I still check Wikipedia as a back up.
Any book should also tell you things like not mixing goldfish with tropical fish (with a few exceptions). Issues with live birth fish like guppies (that may give birth to hundreds of babies - something you may not be ready for LOL)
Then come to the forums with any questions - many of the folks here (not me) have many years of experience and have seen just about everything. Some are also sticklers for being very precise about water changes, using chemicals etc, while others not so much - so after educating yourself you will understand why they give the advice they are giving. For example I try to do a water change once a week on all my tanks but I'm disabled and sometimes I lack the energy to get that done - I do test my water daily and always do a water change on the ones with the worst test results no matter how bad I feel. Just got to suck it up for the sake of the fish - but I may let a couple of other tanks slide for a few days until I feel better.
In my opinion fish are a LOT tougher than some people think - if you thing of what they go through in the wild - it's not always perfect PH and water hardness - they may experience a lot of fluctuations but they adapt. The problem with aquarium fish is that if their water is bad they can't escape it by swimming somewhere else.- so it's up to you to provide the best conditions you possibly can.
Hope your tank helps your child - but make sure you read up on how to cycle your tank before you ever put fish in it. There is an excellent article on the forums about how to do this using household ammonia (no detergents) that takes about the average 4 weeks or so before you can add fish - but you have to have a stable tank before you add fish even if it frustrates your child.
Best of luck and keep us posted.