YOU can always find a cheap tank on CraigsList.
Maybe you can, but I have looked for a year now and everyone here is asking retail value or more (rich people that get a bunch of fish and a nice tank and then dont cycle it, thus killing the fish and making the people angry, causing them to feel the need to get every penny back).
Glofish would be super cool for him, not sure if lights have anything to do with his relaxation but if you turn the lights off in the room and put a black light over the tank the glofish will glow brighter colors, however many pet stores sell 3g glofish tanks that are not suitable for glofish to live happily for a long time, I would give them a month tops in a tank of that size, especially if filled with as many fish as they show on the box. (my 10g box showed 42 full size guppies on it, but would be good for about 8 of them).
Neon tetras are very common and cool looking. They do good in groups of 3 or more, a couple guppies and a school (fishy term for group) of neon tetras would go good in a 10g.
Bettas are really cool, I have a 5g with my betta in it. I think he is a double tail, but I could be wrong. They can be fast when they want to be, but tend to be on the slower side. After the first week or two they really get interactive with you. Mine follows my finger as I move it around the outside of the tank. They will go up and beg for food as mentioned before. Not a huge bio load especially compared to goldfish. Mine happens to be very mellow, but some are very aggressive (that is why any good pet store will have them separated from sight of each other or they will just flare all day).
Also mentioned above were dwarf gourami. There are several varieties in many colors. My favorite is a powdered blue one at the lfs (havent bought him though) and dwarf flame gourami. But honey gourami seem to be very calm, and many people like them. I dont care much for their single color plain look, but your son might.