Leghorn rooster with infected butt?

Hi guys! Warning.... GRAPHIC PHOTO. This is my favorite leghorn rooster. We dust our birds regularly for mites and worm. He has no mites that I can see.

Last night I put Larry (the Leghorn) and his ladies up. They all seemed fine. This morning, I went to let them out. Larry wouldn't come out. He also wasn't crowing. I went out again about an hour later and he was laying on the coop floor. I got him out and noticed his butt was very strange looking. I thought something may have attacked him. But upon closer examination, it looks infected. 

I keep wiping the poo away, and it keeps coming back. :-( His skin seems red and inflamed around the area. When I picked him up, he started throwing up. 

I feel just awful. I love this little guy to death. What should I do? Is there a good antibiotic to give? Can roosters get prolapsed vents? I don't see any puss. 

Thanks so much in advance. 

He is acting sluggish and walking tenderly.

Hi guys! Any ideas? :-( He seems to be getting worse. His fanny is VERY red and his feces is coming out a dirty liquid green. Ugh! He lays down and closes his eyes and acts like he's not going to wake up. I'm really debating on putting him out of his misery, but I keep hoping there is something I can do.

Hi! I understand....

I wormed them all two months ago. He is close to a year old..... We don't have neighbors, and he stays in his pen with his three girls all the time. Nothing new in the pen. I don't have any fertilizer here since I use my chicken, goat and rabbit poo. He was acting perfectly fine yesterday evening. I have him isolated now in case it's contagious. :-( 

Thanks!