Another good tip re: Bettas that I picked up from some Betta owners at the pet store is to put a small mirror on the outside of his tank so he can see his reflection. He'll become more active if he thinks he has an "opponent" to posture to. In reality it will be his own reflection, but he won't realize that. A few different people told me that because they are used to sparring with their own kind, this competitive movement keeps them healthier and happier. If he seems like he's getting over stressed, though, just remove the mirror for a while.
I used to do this with my male Bettas (don't have any right now) and they really liked it. One even stalked his reflection like a cat does. It was hysterical, but he seemed really happy.