Nobody likes to be told that any part of them stinks. Sometimes, however, those conversations are unavoidable. If your boyfriend has bad breath, it interferes with some of the most important parts of getting close, so it's better to tell him what's going on than let him think you don't enjoy being near him for some other reason. Tactful and indirect approaches, humor and, if need be, a casual, direct approach can all get the point across without making him feel bad.
The Indirect Approach
Encourage his oral hygiene. You can indirectly give him a hint while addressing the problem itself. Keep a roll of mints on hand and offer him one at regular intervals. If you're with him at bedtime or in the morning, encourage him to join you for brushing and flossing. Remind him that he should do both, but don't be specific about why. If he's sensitive to hints, he may realize what you're trying to say. Even if he doesn't, if you change his habits, his halitosis may cease to be an issue.
The Empathetic Approach
If you must tell him directly, use an empathetic approach. Make a reference to the fact that you sometimes have bad breath, too. Start off with a statement like, “I know this is embarrassing and I'm sorry about that, but...” Recognize that it's probably impossible for him to know that his breath is an issue. Keep what you say brief and to the point.
Point to a Cause
When the moment's right, point to an outside cause for his bad breath, such as the garlicky dinner he just ate or the realities of morning breath. Say something like, “I'm sorry, I can't kiss you right now, that garlic is way too strong!” This will keep him from feeling self-conscious about his breath. Even if it's not the whole story and his breath is bad in general, this will likely remind him that you are aware of how his breath smells and that he needs to be taking care of it.
If you're not anxious about the issue, he won't be either. Imagine how a couple of friends might discuss the issue, with jokes and a generally casual attitude about it. Consider making it a mock point of pride in the manner of a belching contest by saying something like, “That is some seriously award-winning bad breath, dude. Let's take care of that before our next make-out session.”