Keep in mind that one good thing about air pollution is it’s relatively straightforward to mitigate once you’re aware of the problem. PM2.5 filters can be had for relatively cheap (see Smart Air), and you can wear a mask when you’re out on bad days. Unless you’re really lucky, his peers in Bangkok probably won’t make him feel great about wearing a mask, but just having clean air at home will help a lot. Then maybe you can talk the school into filters, plus policies about athletics on bad days. I’d be surprised if they don’t have these already.