It looks like rain rot. Go buy some anti-fungal shampoo, just be careful around his eyes when rinsing off. Most of this gunk is anaerobic, meaning it thrives in places air can't get to. Cut the grease, allow the skin to breathe, and it should clear up pretty fast. I've even known people to use a mild dish soap to cut the crud, seems to work pretty well actually.
If you're using flymasks, make sure you're switching them out every day, rinsing off the old one and allowing it to dry completely before re-using it.
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