Help! What is this?

One of my young roosters I've noticed, has been sleeping a lot. He goes into the coop and just lies down, with his head under his wing. I picked him up a few minutes ago to check him out and see what was wrong and I found this! My flock did have fowl pox a few months ago, but it looked different than this. Is this fowl pox? It's pretty bad. Also, his mouth is really mucusy and kinda foul smelling. Could that be impacted/sour crop? How do you treat that? I put him in a dog kennel and came straight in for advice. What do I do?

 

Okay, so after further assessment his breath doesn't smell bad as I first thought, but his crop is totally empty and his mouth is very mucusy. I'm thinking it might be wet pox? My others had dry pox. I should add, a few months ago, right as fowl pox was beginning to subside here, a pack of neighborhood dogs came and killed a lot of my birds. A wonderful friend gave me 6 of her own laying hens and 6 older chicks. Her flock had dry pox about the same time mine did, but these chicks had never had it. This rooster is one of those chicks, so he probably picked it up from one of my last birds to heal.
So anyways, if it is wet pox, that would explain his empty crop. There is no shortage of food in my coop, and I know he has not been bullied out of food, so he must not be able to eat. Does that sound plausible to you?