Hello,
It does not look like a melanoma but can be a bruise, rash or dermatitis skin. Melanomas are usually >6 mm in size, with irregular borders ,asymmetrical and dark in color. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
From the picture it could easily be a bruise or a rash. That's not to say a tumor never causes a skin reaction over it, but that seems less likely. Maybe it's a fungus. He should see his primary care doctor or a dermatologist.