It simply means that he likes you and he wishes that he was your man other than the other one. Such people are usually not content with the one they are with, or they no longer like the ones they are with and wish that they could have you over the other.
It means that he still has feelings for you. (If you were previously dating and you dumped him, it would make more sense.) It could just be that he is naturally a jealous person. If you aren't dating him, his jealousy isn't your problem.