I throw my hens a little each day on my sandy soil. They then get some grit along with the high carb snack. I use oats, sunflower seed and song bird seed most often.
Hold back on treats to less than 10% of total food consumption...and make them the higher protein ones. You may should even knock of them for a while....as they may have been used to eating only a quality layer feed.
I still provide mine a separate container of oyster shells as they will only eat it if needed. I live on the northern coast of Florida and can get locally ground shells for $2.50 for 10 lbs.
I've used Purina Game Bird layer on occasion, usually winter & molting... 20% protein....lower than Layena on calcium, but I always have crushed oyster shell available. Hens love it.
He possibly could be having a reaction to eating poison of some type. I would think a poisonous snake bite would have killed him right away. Good luck !!
I have both that are equally used....a bucket with 4 nipples underneath (hanging near door inside a large pen) & 3 cups attached to pipe (inside pen fed by bucket outside pen.)
Has to be their feed or they may have intestinal parasites. Better take sample to vet if they are eating nothing but commercial food with not a lot of scrapes & scratch.