Since I don't have alot to work on. I found a Franklin Neilsen born in wisconsin living with mother Leona and step father Edward Greenway in 1930 in Detroit, Michigan. Does this match anything you already have?
Otherwise, if you can please supply Leona and Carl's approx birthdate or age and birthplace. Leona is not a common name and searches on her might find something.
Are there older siblings of Franklin, that might appear on the 1920 census with his parents?