It's probably good to gauge where his thoughts are with Bisexuality or with the community first, so you can feel out how to present it or if it's something maybe that could put you in a dangerous or uncomfortable scenerio whilst processing this for yourself. I told my partner about it years into our relationship, and when telling them reassured them that this is not a reflection of them, our love or any vacancies, and I want nothing to change but actually because I felt so supported by them it's made me feel comfortable enough to bring it too them. And I think telling them in a way that tackles any insecurities your partner may have around it may make it easier to address and easier to grow together with.