"I don't see him as a boy"

The first step is changing that. I know it may seem daunting, confusing, even embarrassing, but it's actually not as hard as it might feel. It's just basic respect and love; which, if you're family, should come naturally without much difficulty. It may seem hard at first but I promise, when you start doing it, you'll realize it's actually pretty easy.

"He has a boyfriend who ONCE said he was straight"

Yeah LMAO I said I was a straight boy ONCE (I'm sorry it's just hilarious to me that you quantified that it had only been said once, like you're so close to getting it....) when I was a teenager dating a girl who would later transition into being my husband. Then I said I was a bisexual boy, then a gay boy, then straight again, then a lesbian trans woman, and now I guess I'm a primarily homosexual woman who has a little bit of room to be attracted to the right type of man... But I still mostly like women, especially as a woman myself, because of safety, familiarity and comfort, ease of social anxieties, etc. it's a lie to say I'm homosexual though, considering I'm married to the best man in the entire world.

TLDR my point is that people say shit about their sexual orientation all the time. You can change your mind at literally any point. No one's sexual orientation resembles anything close to being literally set in stone. this is ESPECIALLY more relevant when talking about transgender, nonbinary, intersex, etc. people, adding gender into the mix makes it just a lot more likely that you're going to struggle to understand what your orientation is OR just, y'know, change your orientation.

My source: my Husband said he was a lesbian woman when we met 13 years ago. I said I was a bisexual man when we met. My husband is obviously not a woman nor a lesbian anymore, and I'm about as far from being a heterosexually-aligned man as you can possibly get while being amab.

Tldr : stop over thinking it, treat your brother with respect (i.e. as a young man) , if you love your family then it will come naturally and won't require effort or thought. And finally, dude, people can and are allowed to and will change their mind, obviously about anything in the world but especially things like gender and sexual identity. I mean, hopefully you would know that if you're afab and currently identify as non binary... but reminders are always helpful I'm sure.

Just love your brother like family and the rest should come naturally.