I wanted to be the one person that said you dont need to write the letter. But, the fact that it took a white man vouching for you to get this guy to lower his arm...well, that just says it all, doesn't it.
To me it's the classic case of colourism, he's brown, so he feels superior to Black, yet it still took a white man to intervene. Bless him, but fuck, it's frustrating. I know you don't want to waste all your time writing letters, but this one is one you need to write.