Look at all the thumbs down on those answers as well, they're wrong. A betta fish must 'sift' air from the water as a regular fish with thier gills, as well as gulp air from the surface. They can actually "drown" when unable to get to the surface to breath. They have a simple lung system that allows them to breath atmospheric air, as we do, but the trade off is they must gulp air to survive. In short, he's fine.
If you're worried however, you can get a bubbler to add oxygen to the tank. You'll need an air pump (get a small one, about 10 bucks), some tubing (price depends on how much you get) and an air stone (under a buck). This ought to help put more in the water, but he'll still go to the surface periodically.
Best of Luck!