"How can I best support my bi boyfriend?" This is by far the best question I receive, and I absolutely love answering it. (Interestingly enough, I've only received this question from straight women and not from gay men. Not exactly sure what that means...)
If you're asking this question, I feel like you're already on the path to being a fabulous partner, and he is damn lucky to have you. One of the most important things about being a great partner, regardless of sexual orientation or gender, is supporting, embracing, and caring for the needs of your partner. It sounds like you're already doing that.
But that's not why you asked the question. You want some more concrete advice from me, right? So here you go.
Ask him that exact same question.
I'm flattered that you asked me. Truly, I am. But I can only give you somewhat generic advice, even with all the additional details you told me about your relationship. The best thing you can do is to go straight to the horse's mouth. Ask him. Odds are, he has a sense of what you can do to best support his specific needs that have to do with his identity. However, he may also have no clue of the best way you can support him. Having never been asked that question, he might not know how to respond. That's not uncommon either. If this is the case, no worries, let's move on a bit to advice number two.
Create an honest and open line of communication.
You want him to know that while he might not currently have a sense of the best way you can support him, this conversation is never-ending (don't say it like that though, because to most people, the thought of a "never-ending" conversation sounds like torture). But he should be able to tell you, if you do something that he doesn't like (maybe invalidate his sexual identity in some way) that he didn't like it, and would prefer if you did something else.
Now, don't go about trusting him willy-nilly. Just like straight and gay men, some bi men are liars, cheaters, and not deserving of your trust. But if you trust him in other aspects of his life, then you should trust him with his sexual identity. If he says he's happy being in a monogamous relationship with you, and doesn't miss being intimate with people of other genders, then believe him. Try your best to not get jealous or nervous if he checks out a cute guy on the street.
I would talk to him more explicitly about his attraction to other genders.
Prior to dating me, my ex-girlfriend was exclusively dating women. Right before dating my ex-girlfriend, I was primarily sleeping with and dating men. When we got together, we decided to be monogamous, with the notion that potentially down the line, we would open up our relationship. Every couple of months or so, whenever we were having a serious conversation about our relationship, we would ask one another how badly we miss having sex with people of the same gender. Since we were both bi, I think this conversation was a little bit easier to navigate. But even if you're a straight woman or gay man, I think it's really helpful to have this conversation with your partner. It gets everything out on the table.
My ex and I would respond honestly. Yes, we miss it, but the urge to sleep with people of the same gender isn't that great, and at the present moment, we would rather be monogamous than open up the can of worms that often comes with ethical non-monogamy. I think ignoring his attraction to people of different genders isn't necessarily the healthiest way to address bisexuality. I think acknowledging that those attractions exist is the best way you can support him.
I'm not saying you have to automatically open up the relationship if says he misses dating men. No, no, no! You have to be open to it too, and willing to try. If that's a deal-breaker then that's a deal-breaker! But if his attractions to other men are too great, it may mean that you two shouldn't be together. And isn't it better that you break up amicably? Besides, you don't want to be with someone who is pining for other people.
Stand up against biphobia and ignorance whenever you hear it.
When you're dating a bi man, you will inevitably hear someone say something biphobic or ignorant. When you do, call that person out. "No, bi people aren't just greedy. That's messed up that you would say that", or "No, I'm not worried he's actually just gay and is going to leave me for a guy. I trust him". He will LOVE you for that. Oh my god, I can't tell you how happy it will make him to see you defend his sexual identity. But even when you're not with him, you should still call out biphobia.
Embrace the hell out of him.
Embrace him for all of who he is. Encourage him to be his most authentic self. Inspire him to explore his aspirations. And don't let what society or anyone else has to say influence how you feel about him.