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Hello. Thanks for the question.
Could you take a picture of this area and post it here?
You can attach it using the paperclip icon and a jpeg image.
That doesn't look too bad.
It looks like a bruise more so than a skin infection or skin irritation. I wonder if he ran into something or potentially was scratching and bruised himself.
If it starts oozing or getting inflamed, then I would worry about a hot spot/ staph infection that would require antibiotics. With this bruise look, I would just use topical treatment with alternating ice and heat packs. That will help it go down and also help with any pain/ discomfort.
This should go away with topical treatment over the next 3-5 days.