your betta migh have an internal parasite that is makin him not eat.
My advice is to first talk with petco or whatever ou have, and tell
them exactly whats happening. My next bit of advice would be to talk
to a non-chain store. For example a store other than
petco/petsmart/pet supplies plus. In my expirience, the non-chain
stores are much more reliable.

Good luck,
Evan Davis