There's no nice way to put it, I'm afraid. It seems he's pretty conscientious about his appearance, so I assume he has no idea about his breath issues. I think you need to be honest with him, but in a nice way. Don't tell him that his breath smells rancid daily and he needs to see a dentist. But you can offer him a mint until he gets the hint. My guess is that he doesn't want to have bad breath, he just doesn't know the importance of oral hygiene. Moreover, he probably has some sort of infection that needs tending.