My 12 year old boy cousin accidentally saw me partially naked... What do I do?

I didn't realize I had left my door open and started changing out of my swimsuit. I heard him walk by, and then I looked over and saw my door wide open. I know he saw me because he was acting super weird the rest of the day. Should I say something? Or just leave the situation be?
My 12 year old boy cousin accidentally saw me partially naked... What do I do?
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