Hi, almost the same thing that you described just happened with my daughter tonight. We where at my aunt's restaurant and my 19 months old daughter was running around (we where fixing up for a family event) and she bumped her head into a table. When I saw this, I started walking towards her and she towards me. My daughter then came to my arms sobbing air as she does when she is about to cry. Now every time she has done this in the past, I have always worried that it takes a little bit of time before she releases the air and actually cries, so I usually push for her to go ahead and cry and let go of the air. So, I did the same this time, but as I was asking her what's wrong and saying it's OK, you can cry, she fainted in my arms and her arms got stiff and her hands got into a fist and rolled back, I was able to held one hand before it close, but did not dare to try to straighten it. I then headed towards the Ice Maker and when I was about to put Ice on her face, she opened up her eyes, looked confused for about a minute and then started to cry and held tight to me. later on (about 20 minutes later) she was fine and went on to play with the other kids. We decided not to go to the emergency room as my aunt explained that the same would happen with her daughter, my cousin, every time she bumped her head; with some instants been a little more severe and scary that others. We are planning on going to the pediatrician as soon as possible to get this looked into.
I don't know if this helps or if you have found anything more about it by know, but if you have any more information, please write me.
Thank you, and I wish you the best with your nephew.