You've identified several problems that trans* people, and especially genderfluid people have, the "neither one thing nor the other" issue. I understand, as I am non-binary myself. And while I was born with male equipment, I have feminine attributes, as well... enough that I am not and never have been considered as a he-man kind of guy... but no one thinks of me as a female, either. There is no way I could pass as female at this time. I would be be labeled "man in a dress" right off.
That said, what I've learned is that it takes extra work and effort to pull off a convincing look as the opposite gender body. Cis-women can look be entirely casual, even disheveled, and no one thinks they are other than female (maybe not attractive, but still female). And the same for a cis-guy.
That means that people like us have to take extra measures to express that side that our bodies weren't born into.
OR we have to become okay with being "reminiscent of" the gender we are trying to express. Sort of like a tribute band. :-) That's what "tom-boys" do all the time. Some are so successful, that they become completely accepted as one of the guys (even when that wasn't their goal).
The solution to that for me, so far, is to incorporate elements of both into my life, and clothing, and mannerisms, so that when I want to, I can just push it a little further either way. As I keep pushing, I get closer to what I am trying to express and people become used to it so they just accept it.
And that leads me into another aspect you raised. If you don't feel comfortable with being his "boyfriend", then there is no way he will be.
None of us wake up completely at ease with these things. Our culture has ingrained it all too deeply within us. We have to push our boundaries, just a little, in order to expand them. We can't do it suddenly, we have to do it gradually.
You might try encouraging him to do serious "role-play" with you. One of those where he can do this over and over, treating you as a guy-friend. And you do your side of the role-play just as seriously. You both know it is role-play and "make-believe"... and that's okay. Over time, what this does is expand both your boundaries and help you get comfortable with acting this way with each other. Before you know it, neither of you will see it as role-play but as your normal interaction.
You might even set up "code words" or alternate names to signal you want to slip into that role-play. It might look like this:
Him: Hi, sweetie, you're looking foxy today.
You: Actually, I'm feeling more Ralph today.
Him: Oh. Hey, Ralph. (fist bump) How you doin' today, buddy?
You: Like a boss.
And, of course, when you are feeling the feminine side, you can just do that side.
The main thing is that these things take time and happen gradually.
Hugs,
Jackie