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A bald spot that occurred overnight could be from trauma (hair got ripped out) or from excessive licking. Cats can do a significant amount of grooming in the hours that we are sleeping, and in the time of one evening we can see bald spots or even skin lesions break open.
At this point, I think it is best just to monitor it. If he continues to pay attention to the spot, or if it increases in size, I recommend having him evaluated by a vet.
I would start with a physical exam and a skin scraping to look for external parasites.
They can also take a hair sample for fungal culture.
If he leaves the area alone, it will still take at least a month until you notice much hair growth.