I'm proud of your dedication to him, he's a beautiful bird. I wonder if maybe working his legs while he is in the sling may be helpful too. I agree with aart, some carpet or rug may help you out too.
Wow, Kailey, I think we are seeing a future veterinarian or physical therapist in action.
It is so good to see Brownie holding his head up and eating wet feed from a spoon. I use a measuring scoop to hold water up to them to drink. He is trying to flap his wings to move around as well, so a good sign. Is he moving both legs and wings? You are doing a great job with him. So glad that he is mellow. He might get more feisty when he gets to feeling better.