I agree get a good poultry vitamin supplement and add it to there water you can give them some of this water first off via syringe down the back of the throat to start them off.
I had a very similar situation a few months ago as I had a late hatch of Birchen Marans and the weather turned to bad to put them outside then we go hit by a serious fox problem due to the flooding in the area and the foxes all moving onto the mountain where I live. So all my 50 + birds had to be kept inside stables and my Birchen Marans had to remain in there brooder.
Turns out despite good feed and lots of fresh veg etc they weren't getting enough sunlight or enough of certain Vitamins and minerals they just get from scratching around the place. I'm lucky here as I have a good vet who's into chickens so on his advice these guys ended up on a vit and mineral supplement in there water and grated cuttle fish bones for a few weeks as we couldn't get any oyster shell.
Anyhow it saved these chickens and they are due to go outside again shortly once the runs have been re fenced as the foxes dug down 4ft to go under the fence into the run and killed 20 birds one night because the storms broke the door on the coop that night. Not a nice thing to find in the morning.
I also fed them cooked chopped hard boiled eggs, loads of green leafy veg in the form of cabbage and kale, dried seaweed and nettles, tuna, white fish and salmon as we can get it locally here. All my birds look great now and the drunken stagger and backwards shuffle stopped within a week with most of them.
They will stay on the vit and mineral supplement about once a week it go's in there water for a day and I keep a very close eye on them to make sure it doesn't happen again.
