Let me begin by saying that this is the opinion of someone who is in no way shape or form a professional in these matters:

Any disorder that might make it difficult for him to handle crowds or excessive sensory stimulation.

that would be my best guess.

It could be chemical sensitivity, at least perceived. That may lead to a parent choosing a more rural area feeling that it's more "pristine."

But since I deal with these folks, that is the only thing that jumps into my mind.

I'd guess it's either a crowds or a closed spaces kind of thing. In big cities, even when you're outside, between the buildings and the people it feels enclosed still.
Although it could be a chemical issue, like danf suggested. I'd think an allergy to common perfumes or deodorants would be pretty rough in a city, where people are always around. Or car exhaust./

I think I was lumping crowd/noise sensitivity in with autistic spectrum disorders, as uninformed as I am on that stuff.

Hadn't thought about chemical stuff. That makes a lot of sense too.

Thanks for theorizing with me and not jumping down my throat!

Schizophrenia has a well-known association with urban environments (although primary risk factors remain genetic).

There is a great deal of literature showing that people in cities tend to have higher (reported) incidence of numerous other mental disorders ranging from depression and social anxiety to borderline personality disorder.

Isn't schizophrenia usually diagnosed in the late teens, at the earliest? (Don't know, asking honestly.) I'd be surprised if her kid is older that 12 or 13.

That is so interesting, stilicho. I really had no idea that rates of mental illness change with type of environment. That makes me think of chicken/egg questions (do urban environments cause stress and wear down coping mechanisms? Or do people with these sorts of difficulties seek out or feel more comfortable in urban environments?)

"I'm really curious about what that could be, but we haven't gotten chatty enough for me to pursue it further." I find the best thing to do in these situations is to ask directly and politely. If someone is willing to bring a subject up then they a likely to be open to straight forward follow up questions.

In big cities, even when you're outside, between the buildings and the people it feels enclosed still.

Just this evening, I decided to cross the street to avoid walking next to THIS BIG WALL that rose up right next to the sidewalk and gave me a mild case of claustrophobia.

I crossed the street to the side where I had five feet on my left between me and the parked cars, and ten feet on my right between me and an apartment complex, and I felt much better.

If he's still in school, it may also be something where urban schools don't have the resources to accommodate him in some way. But that's assuming that "moving to the country" means "moving somewhere more affluent," which is not necessarily true, of course.

And yes, schizophrenia usually emerges in the late teens, though I've seen research saying symptoms are usually present years before -- but they're usually symptoms that seem clear in hindsight but not so much at the time.

There was also a study that just came out showing that kids with ADHD calmed down considerably after being out in nature (as opposed to being out in an urban environment), and they're doing more and more research on the effects of natural environments on various mental illnesses and finding that time in nature tends to be a fairly good cure-all, so it may be something totally unexpected, too.

My first thought was asthma, which can be exacerbated by traffic fumes.

I was thinking asthma, too.

But then again, why wouldn't she just say "Oh, son's asthma is much better when we're away from pollution?"

also a study that just came out showing that kids with ADHD calmed down considerably after being out in nature (as opposed to being out in an urban environment

Occhi, do you have a link - even to an abstract? We could use that where I work. Our landscapes director is doing a lot of speaking about "Nature Deficit Disorder" in support of our outdoor/nature programming. HE'd be very interested.

Miko, I read about the study on brownfemipower's site, and it looks like the original reporting was done by the ever-fabulous Tara Parker Pope on the NYT Wellness blog; she also mentions a few older studies that support the same idea. The abstract of the recent study is on PubMed.