Most Helpful Guy
You can only tell him straightforward like a colleague of mine told me. It's not necessarily because he doesn't brush his teeth. If he hasn't been to the dentist for years to clean his teeth they will stink no matter how much he brushes them.
Most Helpful Girl
You have to be straight forward.
It may hurt his feelings but he needs to hear it from someone.
The other person may embarrass him If you leave it to anyone else.
He's better off hearing things from you.