He is very cute! I am a huge sucker for the runts and any little guy who needs extra love 
He passed away somewhere around the 5-6 month old mark. I was happy I got as much time as I did with him though, as he was such a sweet little guy. I’m sorry for for loss that’s what happened to my duckling he didn’t grow he only got weaker and passed at least he’s in a better place...Awe, poor little baby
Is he able to open his mouth normally? And is he trying to eat and drink normally, or are you having to hand feed him?
I once had a baby cayuga that was similar, he had a deformed beak and the top portion was bent all the way up. I fed him duckling crumbles soaked in water to make them soft. "squirt" as we called him ate those quite well. But sometimes he would get it mashed in his bill and wasnt able to wash it out. So after eating i would make sure his nose was all clean to ensure he could breathe well.
Unfortunately Squirt didn't grow at all, and while his sibling were 5 months old and feathering out, he still looked like a 2 week old duckling
I would try the soaked crumbles if he is willing to eat. make it pretty mushy and watery. Hopefully someone with more experience will give you some better advice!
Good luck with the little guy! I pray he survives and thrives!
Unfortunately he passed
I believe this happened because the duck had trouble eating and he was getting weak Thank you for trying to help!
It's always so hard to loose one. When they are that young it doesn't take long for them to get weak. At least there is no more suffering for the little one. 
