Awwwwwww, you wonderful person for trying to help this duckling. I'm probably wasting your time by posting this as I'm not a vet and won't give you any sound advice, although I would have thought feeding little and often???? Your best person to answer this for you is Lankeela, s/he knows an awful lot about this type of thing. Hope s/he comes on here and helps you out.
Are you planning to keep the ducky if s/he survives?
Will you let us know how you get on? I wish you every success!