My cat just will NOT eat the adult food we're giving him?

He will not starve to death if you don't give him the food he wants. He'll eat when he's hungry. I think you should make him eat the Halo food, or return it if you can. Then, try another good food like that. I feed my cat Castor & Pollux, and they make a food called Organix too. Good luck, and don't worry about him not eating. My cat ate Purina One for kittens his first year too, and didn't mind the transition at all, but I wasn't giving him the choice. :o)