@ Miss Lydia: I think you are right, maybe it just needs more time to stretch out. I don't know how it actually works with that kind of ducklings, whether there occured a mistake during incubation or if it has genetic disposition or maybe it just hatched a day later than the others and that delay forces the organism to put more energy into motion than growing.
Is it true that ducklings with this colour pattern turn out to be barred? I found pictures on the web that say so.
For him I mixed the duckling pellets with water and ground them between my palms. That created lots of particles small enough to eat for fuzzball. But now it can swallow also the larger pellets. I am so happy that it improved so much despite the bad climate.