Hi
Leighnicki
Do you only have one duckling? It needs ducky friends -- a quail even if it weren't a male traying to mount your duckling, isn't enough
What are you feeding your duckling. DUcklings need specialist duckling starter food. By 3 months they can go onto duck pellets or crumbles, But make sure you are feeding duck specific food and not all flock which does not meet the nutritional needs of ducklings.
I suspect your little duckling has a nutritional deficiency leading to delayed feathering and slow growth. As well as only feeding your duckling specifif duck food, can you get a bottle of rooster booster vitamins from your feed store [or on line from Tractor Supply? Can you also sprinkle nutritional yeast or brewer's yeast onto the duck food -- 1 tablespoon per cupful of duck food
I would not give any treats at the moment -- just get duck specific food into this little fluffy until it starts to put on weight. You might monitor its growth by weighing every week.
If this is an only duckling, please urgently get two or more duckling friends for it.