Does he still eat and drink? Is he passing "pellets" or Diarhea? Is he passing any blood? Do you have alot of bird droppings around the feed area?
I have lost cattle to Cocidiosis, usually it acts fast, they usually die before losing too much weight. In cattle, they stop eating and they pass some blood since the intestines are being perferated. There was a feed mill on Oklahoma that got alot of cattle sick a few years ago because the feed mill had a large amount of wild pigeons crapping all over everything, they were infected with Cocidiosis and they contaminated the mill. This caused people to feed Cocidiosis to there cattle.
I have seen cattle lose lots of weight before and no matter how much you feed them, they wont gain. This can be caused by a disease attacking the intestines and causing them to thicken, when the intestines walls thicken, they stop absorbing nutrients and no matter what you feed, they will starve. Have personally had this in cattle and when blood tested, they test positive for Bovine Leukemia or Salmonella. In cattle there are several "wasting diseases" that have nothing to do with BSE / mad cow or CWD....just the symptoms are similar.
I know deer are not cattle, but they share alot of the same diseases. When my grandfathers family first settled in Indian Territory there were no diseases in cattle, every year or two you hear of a new one. I bet most of you have never heard of Anaplasmosis? Once a cow gets it she will most likely die in a few days if not treated. It is carried by vectors and contagious. My grandfather swears that he never saw a case until OK wildlife introduced whitetail deer to our ranch in the 40's or 50's. He swears the deer brought it in. I am not even sure deer can get it.