Light sensitive betta?

Bettas are naturally black water fish so extreme amounts of light can scare them. He may also just afraid of his new environment and should get more comfortable as time passes. You could put Indian Almond leaves into the tank somewhere to give the water a tan. This also helps the betta fight diseases and live longer but it isn't necessary.

Bettas are native to swamps I believe and don't get a lot of light. At least not enough to see each other as well as you are probably seeing him now. This is why their scales tend to shimmer, so other bettas can see one another in the low light. What types of lights are you using? My guess is its that or the newness of the tank. Both can stress a fish like that. I had a Betta who got a bad case of fin rot right after I switched lights on him. Had no idea what was causing it until I mentioned it to a well informed employee at my lfs. That's what she told me. Sure enough, I swapped the lights right back and he was back to his normal self and begun healing within a couple days.

Give your tank some floating vegetation - even Riccia will work. Keep in mind that freshly bought Bettas come from conditions in which most of them are kept devoid of more stimuli than food and water changes. Plants, light, water flow - it's all new to him. Give him a month, you won't recognize your fish.