If that is all the bird wants, then let him eat "cake" errrr sunflower seeds. I would give him the option of other food at the same time but unless you plan on force feeding the bird I really don't see how you are going to make him eat something it doesn't want. You stated you haven't had him that long, maybe the sunflower seeds are the only food it recognizes at the moment and in time he will come to trust you and eat other food you present to it.