First off, that's an impressively good tank size, and good on cycling it before getting your fish. Make sure there's a lot of plants and hiding spots, some fish might feel intimidated with suddenly having a huge expanse of water especially if he was in a tiny cup for a long time. If he keeps hiding and not taking food, he could be one of those bettas who might prefer a smaller tank (it's rare, but it happens). He could also be ill from being in the pet store.
Please fill out this so other people can have a better idea of what's going on.
Please fill out this so other people can have a better idea of what's going on.