Rooster Not Eating/Drinking and Falling Asleep Standing Up

I have a four-ish month old Rhode Island Red rooster and four days ago I started to notice he was out of sorts. He was quieter, mellower than was normal for him. He stopped eating and drinking, and he's been falling asleep standing up for the past few days. He doesn't crow anymore, though sometimes he looks like he's trying to (opening his mouth, extending his neck) but no sound comes out. I looked him over and there aren't any obvious signs of injury. I've been hand feeding and watering him since I noticed he wasn't taking care of himself, but he's not getting any better.

Any suggestions as to what might have caused this and what I can do to get him to bounce back? I know it's a long shot, but I'm still hopeful.