Oh, nooooo....
First of all, this has to be very painful for you. I'm really sorry.
I don't know if I advocate hypothesizing about him being molested, but it does seem that you have some cause for concern. I applaud you for initiating open discussions about it. Provided you don't make accusations, if I were Mike and if I were struggling with my sexuality, I would feel comfortable (under the circumstances) talking to you based on the way you describe your conversations. Sounds like you're doing the best you can, and going about it in a way that seems fair (despite your pain).
As much as I'd probably do the same in your shoes, I am not of the opinion that it is your job to bring him to terms with "the truth about who he is". If he hasn't been active seeking help, despite the resources you provided, then he just isn't ready. Think about what a huge step that has to be for a man who has always identified himself as heterosexual. Although my ex-husband questioned his sexuality at times, the way I can relate to your question the most regards my (permanent) husband, who has refused to talk about or speak to his father for the past 17 years. As much as I know that there is unresolved hurt that he carries around, it is just NOT my place to push him to resolve it. He'll do it when he's ready. I'm a 'fixer' and it sounds like you are too, but in my case it would just be hurtful for me to push it. Your case is a bit different because it definitely impacts you if he decides to pursue gay relationships later rather than sooner, and he does sound as though he has a hard time with emotional fidelity. That being said, I can't say I would do much differently in your shoes.
I don't know -- it's a really tough call. Has he been evaluated for bipolar? He doesn't exactly sound bipolar, but it would be something to consider.
Good luck to you