The unfortunate thing to me is that for bi men, there are no characters in the roster that makes for good representation.
Like, Mercy, Mei, Pharah, D.va, Ana and Brigitte all would be positive examples for bisexual women in canon.
But every male character in OW right now is either evil, racist or falls into a stereotype. Namely McCree and Genji.
The closest there is for that is Reinhardt (who is a overt flirt with Ana, and therefore propagates to the stereotype of the flirtatious bisexual man) and Lucio. (Who contributes to the trope that being bisexual means you are some kind of creative artist) And even so, it can be done poorly so easily with both of these characters.
To answer OP’s question, I feel you should make your character’s sexuality apparent from (pretty much) their introduction, preferably without making them a stereotype. Steve Cortez and Sir Hammerlock are good examples, I feel. I vastly prefer Bioware’s method to Blizz’s method.
The one time Bioware revealed a established character’s sexuality, it was terrible. Kaidan was a originally straight character that got changed to be bi when Bioware realized that they had a entire species of bi/pan women, and not one bi man.