I don't think there's one correct answer here, and you certainly aren't obligated to come out to your boyfriend (or anyone) if you don't want to, but I do think there's a lot of value in the sort of intimacy that talking about the finer details of one's identity can bring to a relationship. I think it's all right to come out in whatever way feels right or meaningful to you right now, even if there are things you're not certain of; you can even tell him "here's how I feel for sure, here's what I'm confused about, here are things that feel like they fluctuate from moment to moment."
If it helps at all, several years ago I was writing an advice column about coming out and I came up with a formula of sorts for how to structure that conversation, and I still like it and think it may be a helpful approach. Sometimes when people imagine coming out, they think of telling someone "I'm [identity]" but don't have a sense of what else they want to say.
The structure I like to lean on has four points:
- the identity name
- your definition of that name or term
- why you wanted to come out to that person in particular
- how you'd like them to change their behavior, language, etc. where you're concerned, or any other requests
So when talking to your boyfriend, you might say something like "I wanted to tell you I'm genderfluid, which to me means [what it means to you]. I wanted to tell you because [you want him to know you better, you're excited to share part of your identity with him, whatever you're feeling], and I'd love it if you could [learn more about this identity with you, use different names or pronouns sometimes, etc.]." Obviously this is just a bare-bones script, and it's something you could use or adapt however you want, but I wanted to throw it out there in case thinking about a coming-out conversation in this way would be helpful.