IMO, the tank is too small for a betta and tetras. It's also quite possible the tetras are harrassing him, or fighting among themselves. If the tetras didn't go thru quarantine, it's also possible they brought a disease with them.....and the Betta is sick. For his safety I would separate them.

Here's one discussion of tetras with a betta.

Also, check out theBetta Forum for more advice.

did you cycle the tank before you added any fish?

Excellent question Nina! That slipped right past me. :)

Kyra.....you have to let a tank cycle for at least a month. Have you been testing the water?

i wish more pet store employees would explain the concept of tank cycling. so many people buy and set up tanks without knowing how important it is! not saying this is the case with the OP but far too many fish die because the tank hasn't been properly set up.