I don't think there is any "good way" of telling him.
You two are in a relationship and their is (suppose to be) trust and you've compromised that trust by not telling him, and the longer you keep this from him, the more he's going to resent you for it.
Just because one is bisexual (or gay) doesn't mean they have to act on it. If you feel you love your bf and that you feel a sense of loyalty to him, you won't have to act on it.
Ask yourself this: What's more important to you....your relationship with your bf, or living a bisexual lifestyle? Only you can answer that.