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Hello and thanks for trusting me to help you and Kai today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
If it just appeared today and doesn't seem to be bothering him, it is probably ok to just keep an eye on to tonight.
It would be helpful if you could send a picture.
Thank you for the photo.
It looks like it may be a trauma of some sort may be a possibility, but with the surrounding hair loss, it could be a dermatologic lesion. If it is not resolving in the next day or two or if you start to see other lesions like that popping up, I would definitely have him examined. If not a trauma, like an abrasion, it could be an excoriation (if he is scratching or rubbing his head). Flea bites, mange and ringworm would all be possibilities, so if not improving, I would have him checked for those.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
Revolution would decrease the likelihood he was dealing with fleas, but ringworm is a fungus and would need to be treated differently.
Here is a link with more information about ringworm and how it is spread and treated.
You are welcome.