I have an ASD son as well. Seems its his dad though that has the fish tank problem~:grinno:
Your tank size will be the major limiting issue, but you know that, so there isn't any reason to beat that horse anymore.
I would start with a betta. Its not a perfect set up, but its not the end of the world either. It is what it is, and you are now educated on the needs of tank maintenance. Keep track of water parameters and things should be okay.
Not knowing where you son is on the spectrum, I've found with my son, that it is in the building of a new tank that he gets alot of enjoyment. Admittedly, his ASD is not as severe as some kids and his socialization is improving, but he really enjoys the "science" behind the tank. "why do we wait to put fish in" "what do we need to make sure the fish stay healthy" "how do we plan to do those things". extending that out, we let him talk to the "fish guys" at the store, explain what we need to get for the next step in the plan, let him engage with people other than family.
Again, you know as well as I that there is no one path for ASD and what can be productive for some isn't scratching the itch for others. Best of luck, and if you want to talk, the PM is open.