Child Is A Fussy Eater

My son is a picky eater. He?s a little underweight for his age, and also allergic to milk. He likes hot dogs, so I buy him the 98% fat-free turkey dogs. My husband thinks he should get regular hot dogs because he is a still a young child with no health problems ? and he needs the fat for energy. He also thinks he doesn?t need fiber. What should my son be eating?