If his name is Kevin, you’ve got it made. Otherwise, you might want to leave it out of the poem.. Seriously, there isn’t much that rhymes with heaven. You’ve got leaven, Devon, eleven and replevin. None of them would seem to work that well in a romantic poem, although Devon is a nice place. Replevin is a legal action, so best to stay away from that. If you’re willing to have a really awkward rhyme, you might be able to use leaven (which is stuff that makes bread rise, like yeast. I don’t know — his/her love is like the leaven in your life. Not great. Maybe it would be easier to find rhymes for “paradise”.