Welcome to the club that none of us wanted to join. In all likelihood he has met a man wants to be with. Sean who is gay and who has a section here has explained it very well."Bi" doesn't mean that there is an equal amount of attraction to either sex. "It can be a small percentage in fact to women, enough to date and secure a wife but not enough to keep him from wanting a man. Some will sneak around cheating on their wives for years.

It took my "bi" husband about 30 years after he told me he was bi  (after we were married) to feel comfortable enough to exit the closet and tell me that he was gay, was moving out and wanted a divorce.
If he's asking for a divorce now because he wants to be with men, it doesn't matter what he calls himself, whatever he is he isn't for you.

This sounds harsh but it takes two people fully committed to each other to have a chance of a happy marriage. He's telling you that he's not going to be. Break the news to the family and friends who will be your support system and consult an attorney to protect your interests.