I was in a similar situation, except I was the somebody with a partner. I was in a relationship, and I wasn't happy. It wasn't easy for me to end the relationship for reasons I won't get into it. I had a best friend that I unexpectedly fell in love with, and despite him feeling the same, he remained respectful of my relationship. He was still an escape for me, though, whenever I was going through tough times with the guy I was with and needed someone to lighten the mood. Eventually, I ended my relationship, and then got with my best friend later down the line.
Despite the guy you're seeing not being happy, I can assume he's still emotionally attached to his partner; therefore, finding it hard to let go (otherwise, what's stopping him?). Either that, or he's not being honest with you.
