It is odd isn't it. Discriminating against black models in order to not cause other people to be reminded of racism.
More, not using the black kid for one particular hoody, because otherwise it looks like you're ridiculing him.
However, let's recap. We are talking about the image choices on H&M's online catalogue. If you mouse over the boys' clothes on the H&M site, images of the item as worn appear. The same models are used over and over, which makes sense. The exceptions are probably ones where none of the photographs turned out well.
I imagine all the new clothes were done on the same shoot, however long it was. I think it is reasonably possible that on the day, multiple children tried on that Coolest Monkey in the Jungle hoodie and were photographed doing so. If that's so, I could see why the implications might not have got picked up on at the time.
Then, the photos got developed, and (presumably) a team set to sorting out the best shots for the online catalogue. At that point, someone thought that was a good arrangement to have for that set. White kid in the Explorer hoodie (very colonial) , white kid in the animal print hoodie, and black kid in the jungle hoodie.
That was a shite decision, wasn't it. At best, no cultural awareness. At worst, someone thought it was funny.
No child needed to be told they couldn't wear something. The images could just have been dropped to the floor, along with all the ones where the model shut his/her eyes, looked in the wrong direction, or had something growing out of his/her head.