Its a big investment but its well worth it. If your serious about it i wouldnt get anything under a 100gal tank.. Like someone said above the smaller the tank the harder it is to keep everything in range..
I had a 135 gallon for a few years but had to get rid of it because we moved into a smaller apartment. Your son will get a kick out of all the snails and crabs you can add to the tank
The site below has a ton of info to help you start your own tank..Also the members there are extremely helpful