On the weekend.
J is disappointed that I didn't mention this earlier. So....
On Saturday, J went with his best friend and his best friend's family to the playground at the school. While they were there, J slipped and fell off the climber, flat on his back on the gravel below.
He was quite upset when they brought him home. He was having trouble breathing and his side and back were noticeably red. At one point, he thought there was something wrong with his heart but I pointed out that he probably strained a muscle in his back trying to stop the fall and that was what he was feeling through to the front of his body.
He's still a little sore but the redness is gone and his breathing is fine so I think it's safe to say that he's survived.