Corn is not really good for them to have too much of, just a little bit every once in a while as a treat...
Have you had them all since they were ducklings or did you get them already grown? If as ducklings, did you supplement them with niacin or brewers yeast for their deficiency? Could be niacin deficiency, or vitamin deficiency (look up wry neck/stargazing), or it might be neurological... or injury... shine a bright penlight into each of his eyes, check for even and reactive pupils...
@Amiga any ideas, please???