I'm not positive with bettas but IME
it could be one of two things.
If it progresses to the rest of his body, sounds like it could be a case of dropsy. You will know when it is dropsy because his whole body will become "pineconed", where the scales stick straight out.
Other thing it could be is a bacterial infection. Spots of his body will become inflamed. More than likely, this is the case. What did you use to treat him with? What are your water parameters? When was your last water change?
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