Whiplash is, as memory serves, the result of spine extention, noticeably in the neck area, and can happen even at relatively low speeds; whiplash is pretty common amongst headbangers if you can believe it. In this case it was probably caused by the abrupt deceleration of the trunk whilst the un-braced head continued to travel forward, causing the spine to stretch beyond it's normal limits until the muscles and ligaments did their job to kept the kid's head firmly attached to his body. If you have troubles visualizing what I'm saying, mime knocking on a door. Your hand is the kid's head, your wrist the neck, and your forearm is the spine. Keep knocking for about 10-20 seconds and your wrist will start to hurt; you don't even have to knock 'hard' to cause discomfort (but the harder you knock the quicker and the more intense the discomfort). What you're experiencing is the most common form of whiplash.

Like children that never grow up, that you adopted from his parents because they had enough of his shenanigans.

One figures dumb shit like this might happen with teens or young college students, but past an age it's sad. It's even worse the father was involved and dared him. I have a strange feeling if the son does stuff like this father is going to have "boys will be boys" mentality, but after the trip to the ER maybe they've both learned a lesson. At the very least if you are gonna do dumb stuff make sure you are getting paid well.