I agree with blompot --- you need to tell him, because this probably affects other aspects of his life as well. Remember that we all notice a bad smell when we first meet it. If it is persistent, such as when we ourselves are the source of that bad smell, we stop noticing and smelling it. I remember when I visited the volcanic area of Rotorua in New Zealand, with it's thermal springs and geysers, there was this enormously strong and ugly smell of sulphur in the air, and I wondered how anyone could bear to live in the town. Within 10 minutes we no longer noticed any such smell at all.
So tell him, tactfully and kindly, that he is a charming and loveable man, but that, and you recognize that it may be something he can't notice, he has really bad breath, and should try a different tooth-paste and some mouthwash, as well