If you were friends with someone with bad body odor, would you tell them, and if so, how? or is it better to not tell them?

I have a good friend (opposite sex, platonic, no sexual intentions) who has bad body odor. To be specific, it’s like un-washed underarms. I don’t understand how he does not smell it himself because it is very strong. He’s a fun person but this smell is gross.

Should I tell him, and if so what is a way to say it tactfully? Or should I just leave the topic alone?