I was in a somewhat similar situation recently and it was hard on both of us. I met a girl who was married and poly, like me, and I really wanted to be friends with her, since she was someone who could relate to my lifestyle.
But she ended up having feelings for me that I did not return. Since I haven't had much experience with women, I was torn between wanting to pursue the sexual side of the relationship for the experience, but not being attracted to her.
Didn't take me very long to figure out that wasn't going to work, but in the meantime I inadvertently sent mixed signals.
Part of my thing was that I did not want to lose her friendship.
Well, it ended up with very hurt feelings on her part. Mind you the whole situation happened within the span of eight days, but I felt afterwards like I should have been more honest with myself first, and that would have allowed me to be more honest with her.
Although she told me we would still be friends, it didn't work out that way. As soon as the romantic possibility was off the table, she stopped calling and texting me. She said it was so she could work through her feelings, but I also suspect that she was not interested in "just" friendship.
So what does this have to do with you?
I think it's better to put all your cards on the table and let the dice fall where they will. Accept the possibility that you might lose the friendship as well as the romance. Not going to lie, that's going to suck. But otherwise you're leading him on. At the very least, tell him that you don't see this relationship being forever.
That actually reminds me of another story from years ago. I was dating this guy, it was fun, but I knew it wouldn't last forever. That came up once in conversation, and he broke up with me two days later. He basically said that, although he didn't know whether I was "the one," he said he couldn't be with someone if there wasn't at least the possibility of marriage at some point.
I had to respect that. It sucked to lose the romance, but it isn't right to hold someone back from working towards what they want in life, if you know deep down that you're not it.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker