Most Helpful Guy
First off I think him not responding to your messages means he's actually doing something with his life which is good than being clingy and waiting to respond to your messages as soon as he gets them. Second I don't think he responded to your message in the morning because he probably hung over from last night.
Most Helpful Girl
I think you should call him out on it. He's probably thinking its not such a big deal because you have accepted it. That way if you tell him it bothers you and he does it again you know he doesn't really care about your feelings. Then you can let him go. You don't have to call him out harshly though, just do it in away that lets him know you're serious.