Telling someone something that is potentially embarrassing puts you in a difficult place. If your boyfriend, however, has breath that stinks, not saying anything can impact your relationship and, possibly, his other relationships with friends, family and coworkers. It's also tough to hide your repulsion when faced with truly unpleasant breath, but in that moment, speaking impulsively can be interpreted by your boyfriend as mean. Even though you have his best interests at heart, it's important to be tactful in explaining your boyfriend's bad breath with him.
Choose a private venue to discuss your boyfriend's breath with him. One of the primary goals in pointing out a difficult fact is to do so without an audience. It's also important to choose a time when neither you nor your boyfriend is experiencing significant stress or conflict. This can change the way your explanation is received, through the lens of other, potentially unrelated problems. Prepare yourself emotionally to be as calm as possible, so that your boyfriend isn't given the impression that an argument or breakup is about to ensue.
Communicate the problem clearly, without adding inferences of disgust or other reactions to your boyfriend's stinky breath. Using "I" statements to explain your feelings about the problem can help reduce the chances of your boyfriend becoming defensive. For example, say "I feel angry when you don't take care of your breath because I get the impression that you expect me to tolerate it, and that is not my expectation of a relationship," or "I feel embarrassed when you don't take care of your teeth because I feel that you don't care that it makes me feel queasy." Explain the problem factually and how it is affecting you. Avoid placing blame or accusations on your boyfriend and do your best to remain relaxed, face your partner physically and express concern for his health.
Explain your concern briefly for any possible health conditions that may have led to his bad breath. Aside from the influence from certain foods, bad breath can indicate serious health problems including periodontal disease, infection and impaired digestion. Tell your boyfriend that you are willing to support and accompany him to seek medical advice to determine the cause of his bad breath. If his bad breath is caused by frequent consumption of common breath offenders such as garlic, speak with your boyfriend about taking measures to reduce the intensity of the smell. For example, say, "I know you love garlic, but can you cut back the amount you eat or brush your teeth after eating it?" or "Can you consider not eating garlic if we're going to spend the evening together at home?"
Point out that your concern is rooted in how much you value your relationship. Even if you've reached your breaking point and feel compelled to be mean and poised to "attack" your boyfriend for his apparent ignorance, the most effective approach is to assure him that his bad breath isn't a breaking point. Once you've provided him with this reassurance, determine for yourself the expectations you have for change. Communicate those expectations to your boyfriend and a time frame for taking action, with your support. Be reasonable in the amount of time you are willing to wait for him to enact steps toward change.