Me and my boyfriend are still pretty young, but we’ve admitted that we love each other and hopefully, will keep dating for a long time. We were best friends before we started dating and tell each other everything, it’s so easy to talk to him. Recently he told me a secret, he said thinks he might be transgender to female. I’ve always been super for those types of things and I still love him (can I still say him?) but we’re still so young and it’s just he never showed any signs or clues. In fact he’s always acted super masculine. Wearing muscle shirts, being taller than me, and taking pride in having broad shoulders and growing more facial hair than other guys in our class.
Sorry if I’m being offensive by saying him too much, but he’s not officially positive. He says he wants to explore some stuff but he’s already looking at new names and talking with some kids in an LGBT club that we were friends with before. I accept him, really, and still love him but it’s just moving so quick, he told me this just last week!?
What do I tell him? What if it’s (please don’t give me hate for this) just a phase or something he’s going through because of how young we actually are, I’m assuming younger than most of you on this website.
LemonVibe 
I think it’s time to educate yourself a bit. (I don’t mean that rudely so I sure hope you don’t take it that way.) Google lgbt family and read up on how to be supportive.
Now on to you. It’s OK to love him like he is. It’s OK to need something else than this relationship as long as you let him know and arn’t a jerk about it. You are both growing and changing now and will change your minds about so many things. I am a different person than I was at 13, 20, 25, 30, and 35. Part of being human is learning and growing.
Internet hugs for you both
I agree with the comment above, because, as you’ve said, you don’t know if it’s a phase or not. Learn a little more about it. Look at people like Gia Gunn, a drag queen who transitioned very recently. Listen to their stories. If he’s thinking about transitioning, it’s important that you, being his SO, know what to do in support of him.
Perhaps it would be good for you to talk to him about it. After all, it’s a pretty big decision, transitioning. Ask him a little bit about how he feels, and has he felt it for long. Make sure you make it clear that you’ll support him. As unfortunate as it is, the world still doesn’t accept transgender men or women fully, and it’s likely that, if he does go through with it, he’ll be faced with an uphill battle. More than anything else, your role as his significant other is to support him in this.
As for pronouns… I’d be inclined to keep referring to him as a ‘he’ now. When he officially decides he is transitioning, or that he is a woman, then refer to him as ‘she’. And, please, make sure you never refer to him as ‘it’.