Does his tank have a heater? if it does I would try to get the temperature between 78-80, 75 is too cold for bettas.
If your tank is small then you need to do daily water changes or your fish could get really sick.
I would recommend you get some test strips for your water, the ones I have test for Nitrate, nitrite, hardness, chlorine, alkalinity and pH so they are super easy to use, I would also get a test kit for ammonia.
Are you using a water conditioner?
Hikari foods are good, but I would try some frozen bloodworm's to get him started eating, you can buy them from most pet stores, they normally keep them in a freezer near the fish. If he will eat those then you can try feeding him some pellets again, I did this with my girl and now she will eat anything.