Oops, sorry. I must have missed that post!
If it's really small (like a 1.5 gallons or smaller), I'd look into saving up for a 2.5g aquarium kit. They're really manageable and give your fish enough room to move around. Also, it's a good idea to get a heater if you don't have one already. Bettas are tropical and should have 76-82 degree water.
Sorry, I don't mean to lecture!
I'm sure what he's in now is better than the cup he came in at the store. Bettas are pretty durable, but they'll absolutely flourish in warm, roomier conditions. :)
But if he's in one gallon or less, I would change at least 50% daily. When my betta was in a gallon sized bowl, I did 100% every 2-3 days. 100% changes can stress your fish, that's another reason why larger tanks are recommended.