Well, a few layers and corrections later, and this one is turning out pretty well. I've started adding some colour into the shadows, because I want there to be quite a lot going on, given that the subject is high energy, even if the original photo was more neutral. Also, I think I've got the boy's face sorted out pretty well, he's not looking squashed anymore:
And then here's a photo with another layer, and starting to build up the background a bit more. I've also toned down that orange shirt a bit, because I was finding it too distracting in the overall composition. First a added blue and purple, but that really muddied it up too much, so I ended up blending in some light grey, and that really did the trick.