1.He makes no plans
You never know where you stand. He never plans or promises you anything, because he knows that something better can come up any second. By better I mean the one who is more important than you are.
A spontaneous relationship is fun, but being spontaneous and unreliable all the time can be a bit suspect.
This is closely linked to the first point. Even if he plans something with you, he will dump you at the last minute. And he will feel no guilt. Your feelings are not important to him.
The only thing that matters is that he gets what he wants. Sometimes when nothing else comes up, he is content with you. Want to be someone’s second choice?