How

Yes, you can certainly say it. It's grammatical. You have to use the adjective form. With 'Chinese, that isn't obvious. He's so American or She's so French.

I agree, but I can't understand why you say that with "Chinese" it's less obvious.
"My friend Antoine is so French!" (= "so typically French"), and "my friend Chang is so Chinese!" (= "so typically Chinese").
We have probably fewer occasions to meet Chinese than French or British people, but that doesn't mean that there can't be any typical Chinese men.

I absolutely did not mean that, although what I said was very unclear. I was talking about the form Chinese being used as both an adjective and noun, thinking mistakenly that my other examples had different adjective and noun forms. But "American" is both (I am an American and I am American). But how about "French"? Is it both? Can you say 'I am a French?' You can't, can you? I don't think