Line manager has to have a quiet and sensitive word, there is no other way. HR won't be interested and would probably cock it up if they got involved, other ideas are either too subtle or too cruel. It's a horrible conversation to have but, speaking as a line manager, it's the sort of thing we are paid to do.
I feel for the bloke, I shower religiously every morning paying special attention to the under arm but can still suffer from bo in the evening. I explored the idea of botox to reduce sweating but just use a specialist antiperspirant and deodrant. Basically it's not always a personal hygene issue but a physiological one.
A guy that works with my missus has very bad body odour. What can be said, in a sensitive way?
My Mrs also has the same problem. This wouldn't be a place near Shoreham Airport would it?