I think you are doing a fantastic job with your duck! He looks pretty content and not distressed by what I could see. I'm sure he must be a bit frustrated but I will bet that with time he will recover. Nerve damage takes time to heal as does the air sac issue.
Do get some of that B-complex that
@Miss Lydia suggested because its just plain good for him and will help in healing. You can get it a Tractor Supply and you'll want to put 1 ml over a treat that he will eat right up. You can do it a couple of times a day. I wouldn't put it in his water though because it gets too diluted in my opinion. Better that he gets the full dose on treats. Here is what the bottle looks like and although it says "injectable" it can be given orally.
You should be very proud of youself for taking such good care of your duck!
View attachment 2030904