I really don't buy into the idea that behaviors are dictated by birth order or the number of kids. I don't think there's an "only child syndrome" unless it's allowed to develop.
I agree with the comment below that we often say "Just a minute" and we wind up making it 5 or 10, which is an eternity in a child's life. But I also think we sometimes drop everything when they interrupt, and then if they don't get responded to one time, they freak out. Some kids think that if they just say "Excuse me" (once or 20 times, getting louder and louder), it's an automatic right to interrupt.
One technique I've seen is that there is a signal between parent and child which indicates that the kid wants to be heard, and that the parent acknowledges it but can't really forget about it either. One technique is that the child puts a hand on the parent's leg (or arm, whatever you choose), and the parent puts a hand on top of the child's. That says "I hear you." Then you move your hand gradually from the child's thumb to first finger to middle to ring to pinky, and when you get to the pinky, you stop and see what he wants. He gets to feel that there is "progress" as you move from finger to finger, and he can "see the finish line" in his mind.
The stomping off with "I hate my life" or crying is designed to get your attention by being more dramatic. If you respond, it reinforces that behavior, which is why it repeats. He is being immature, but he may not know that.
You don't say what you expect him to do to "take responsibility for his actions" so it may be that he doesn't fully understand what that concept means. I'd sit down and work out a system for him, and let him know what the consequences are for having a tantrum about not getting immediate attention all the time. If you implement a system that you can reward, great.
But at 7, he should be used to raising his hand in school and waiting for the teacher. I'd check with the teacher to see if maybe he's feeling overwhelmed with all the kids in his classroom and therefore putting added pressure on you at home by insisting that he not wait for anything since there aren't 20 other kids in the house.