Has he got a mucky backend/runny droppings? try and get him to drink, also some electrolyte (homemade version if nothing else). Stomach distended (= blown up)? Raised temperature? Pale eyes/membranes?
Don't give any concentrate, just good hay and water/electrolyte and get the vet to check him out asap. Can you put him in a separate pen, within sight/sound of the others? Keep him warm if he's shivering.
There is something about chicken feed - my lambs/ewes were going mad for it last year and broke down the netting fence ALL the time to get in.... fortunatley no consequences. My dog also tries to steal from it whenever possible.