That doesn't look too bad. I would first try to pull that crud off (looks like stuck piece of poo over the vent, so you MUST get this off, he will die from not being able to poo). Then I would wash up the area and start looking for lice or mites as either of those can cause irritation around the vent area (that redness you're seeing on his skin, the urge to pick the feathers there), as well as diarrhea. Rarely, roosters can also get vent gleet. Is it sickly sweet smelling? Or just smells like normal gross poop?