Pale comb, lethargic, and dropped wing

I have a black mottled Duccle' roo that is a very sick little dude. I have noticed that his comb had started to become pale over the past couple weeks and I figured it was due to the chilly temps we have had, but yesterday he was staying alone and close to the pen when I would them out. He would swagger over to get water but then just stand in place and "sleep". I can just pick him up without him making any fuss. I have brought him inside to monitor and I gave him a shot of antibiotics that we had for another chicken. He seems to have dried crust on his beak at the openings and when I clean it seems to be somewhat runny. It smells pretty bad as well. I'm uncertain if his crop has soured. I went to check on him last night in the pen and he was too weak to roost and was just sitting by the food bowl. I feel that he may be too weak at this point to fight off whatever he has. Any suggestions on treatment? Thank you!