What is this??!

Thanks everyone! I cleaned his butt more throughly and made sure he was nice and warm.

Looks like the problem is persisting since when I came back from work there was more poop on his vent. I will keep cleaning and try to the coconut oil trick.

Should I be concerned that his butt feathers are not grown in? The whole back of him seems pretty sparse as you can see from the original picture. Maybe this was just an extreme case of pasty butt?