I was thinking if the foot can be laid out even if for now he isn't walking on it at least it will keep the foot in the normal position not curled . And yes Using Poultry cell has come highly recommended on here for struggling ducklings with leg issues. Please keep us updated.
and :welcome
Hard to say it's a slipped tendon. The foot can be placed on a boot so it will lay flat but needs to be done asap. The little one is going to have a hard time getting around as it grows is a vet an option? to determine if it's a slipped tendon or a deformity...