Wow, this is a lot of responses. Thanks everyone. I originally thought ESFP for him, but then I compared him to my ESFP sister (who is basically the stereotype of ESFPs), and they are pretty different. My sister does not have that overt neediness. Nor does she have that seriousness that he has. She also doesn't write poetry (she thinks it's stupid), and she can be a lot more mean than he can.
Here's a question. When he's rambling on, what's he rambling about?
That could give a clue.
He is usually making absurd jokes that hardly make sense. Or literally saying nonsense. Or relating what we're doing to some of his other friends. He likes to tell stories about "what happened at so and so's," or "what he did on X night." And they're usually calculated to impress.
One-on-one he tends to lose the act. When he's alone with me or with my brother (who's in his age group, and is legitimately his friend, not a "friend" by proximity) he likes to start to act "serious." For instance, he recently came to me for advice on which program he should go to. He knew perfectly well where he was going to go, but this was his way of relating and getting attention from me.
He'll also often get angry or hurt about jokes we make about him or his friends, even though they're made in good fun. In other words, when he's not trying to impress or get people on his good side, he gets serious. This is when he's most annoying to me, because he's needy and an emotional drain.