What would he have said?

Did you put details of him in the poem, like maybe he might be able to pick up on subtle clues?

And I think it would be flattering if you could pick anybody to write about and you chose him.

I also don't know what the guy is like.

And it also depends on how the poem is written. Is it admiring or longing, because they can have two different influences.

Guys are probably pretty clueless on poetry and reading into it. Did you guys discuss it afterwards or he just said it was beautiful and that's it?

I think the main question here is, did you want him to know it was about him? If so, why wouldn't you tell him that. Then you could have really found out what is true reaction would have been. And I bet it would have been a little different, but depending on if he likes you in that way, it could be good or bad.