'Thanks, I do keep him quiet, but if I don't help him 'sit up' he tries to do it by himself and then falls over, which is causing him even more stress and pain'
I think it's unhelpful to say anything except that this is a v. v. serious situation. The emu yearling I tried to save, found in a fence -- very similar circumstances -- wouldn't sit still, to rest and heal, because it was terrified of me. Watching it thrash about was heart-rending.
If I had to do it again -- I lost the chick -- I'd literally immobilise it: bind its wound (I used a poultice of herbs), then bind its legs in the familiar sitting position, put it in a box of straw, put food and water in front of it, and see what happens!
Even three or four days of this, providing the 'swaddling' is not injurious, is not gonna do more damage. The bird may be hungry, thirsty, and stressed; but it gives the leg some small chance of beginning to heal.
S.E.