I know that most of us are inclined towards distance, and obviously this kid has serious wheels that looks like he could make a real splash in the 400/800. I also think it would be in best interest to move up in distance very slowly to get the most out of his 400/800 before considering moving up to the 1500/mile. BUT, it looks like to me that he very possibly could have the stuff to, in time, become a phenomenal miler. So much so that I think it ultimately be his best event (like in 7 years).
If you are that fast and can not just win locally, but be awesome NATIONALLY, in the 200, 400, 500, 600 and 800, then why would you even consider running longer races? Of course you wouldn't. I'm sure his workouts are all pretty sprinter-like, and I bet 15-20 miles a week is a lot to him.
That said, off of that he has run a 4:30 and has run a 5K in 16:30ish (I think). And, the only reason he ran the 5K, I believe, was because the coaches really wanted him to to help the team. He hasn't run xc, and he's only run like 5Ks. I'm sorry, but being able to do that is in no way a sprinter, or even a pure 800 runner. That's an 800/1500 guy that just hasn't gotten there yet. Which is great... getting those 200/400 meter times as fast as possible now is not a bad plan. Who knows... in the very long term, he may be able to run a 5K that is comparable in quality as his 200/400s.
Additionally, being that his parents were mid-distance stars, that would suggest higher odds that his genes go that way as well.
Finally, IMO, a person is only going to be the very best in the world at a distance that they themselves are best at (for themselves). So, he may have the stuff to run 44-low (or even faster, I don't know), but if he isn't an absolute pure speed guy like Michael Johnson / Jeremy Wariner in the 4 or Rudisha in the 8, is he going to beat that level of competitor at those distances? Seems to me that they likely would be hard pressed to run 4:30/16:30 even after training for it, but Sumner has already done it off relatively little training.
And that brings me to that every runner eventually wants to hone in on what they are the very best at because if you are the best in the world, that's when you get recognized the most, get paid the most, etc. The mile would be a good event to be the best at, especially with the how the US loves the 1500/mile.
There are some potential roadblocks that could keep this from happening. For all I know, running itself may be 4th on his list of things he likes to do (even though he is so gifted). Maybe he likes playing the guitar, writing code, drawing, or who knows what? And, maybe he has not found much enjoyment in running distance and has no vision whatsoever of being good at anything over 800 meters. Or, maybe he has a girlfriend that he wants to follow to Georgia. And, then, apparently he is from Georgia and has committed to go there. Being that Georgia caters more to sprinters, they likely wouldn't have him exploring distance options (or even thinking about it) at all. And, is there any other coach who would give him a full ride based mostly on the plan to gradually move up? I doubt it.
So, it could turn out to be one of those things that we may never know! I get why people would say, "only on Letsrun would we be having this phenom move up to the mile!" But look at what he's done and his parents, that doesn't scream 200/400/800 to me. It more points (eventually) to 800/mile. I hope he turns out awesome(r) at all of them!