Oh, okay. With bettas, it's really an addiction you can't stop. xD
Breeders do daily 100% changes to their bettas in jars so I'm sure it's okay with 2-4 water changes a week. In my honest opinion, I think 3 should be the maximum amount of changes. There's a water change sticky by OFL, she did a lot of experiments to make up a non stressful water change schedule.
Doesnt it stress the betta out with so many water changes?
It's not really a stress factor.. more of a too sterile one. It is quite stressful, and some bettas are a little more "fragile" than others. A 1g is a great home for a betta, just have to make sure it is taken care of appropriately for a small tank. Anything between 1g and 4g require the same care, water change wise. Small tanks are great, just need a bit more care without going overboard.
Or a 40g, there is a debate on that. Most here just commonly refer to tanks under 5g as "small". Calling a 29g "tiny" or "minute" just seems silly.. which is what nano means. And why am I dragging myself into this? ....
Don't put anything in the one gallon, and don't put anything else into the five gallon! A one gallon tank is a death trap and the betta will not thrive. If your son wants to be a marine biologist show him that his pets need to thrive. A five gallon tank with a sponge filter and a heater kept at 80 degrees would be perfect for a single betta.