I'm worried about my hamster?

His name is Uncle Mellow(don't ask). When I feed him, he always searches around in his food bowl for the sunflower seeds, and those are the only things he eats. I've even tried hand-feeding him the other stuff(I feed him Fiesta Max food for hamsters and gerbils) but he refuses all of it except for the seeds. I'm pretty worried because I don't want him to be malnourished or something like that. Please help!!!