Am I misreading signs or something?

Why not tell him that if he ever breaks up with his girlfriend you would be interested in him? It's not being inappropriate. You're not telling him to cheat on his girlfriend. You're not suggesting that he break up with her for you. You're just letting him know that if anything ever happens, you'd be interested. You can even say, "I respect the fact that you're in a relationship. Don't take this the wrong way," or something like that.