My son wants to wear thong underwear. I am not sure how to react?

My son (13) was helping do the laundry over the weekend and noticed some of my daughters (15) thongs in the wash. He was curious about them and asked about if he can get some? I have always portrayed an open platform about discussing sex and related issues with him and my daughter, but I'm not sure about this yet and I was wondering about some opinions? I wouldn't have expected this question from my son, whereas I always expected it from my daughter. Is this even appropriate for a boy? Although there might be male thong-style underwear on the market, it still is a niche and not common for males to wear such underwear.
My son wants to wear thong underwear. I am not sure how to react?
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