Re-watching the video again and stopping at key moments, it's even more clear that this was a large kid, and you can see (in the background) that the restraint was not fully pulled down over his torso and the lock engaged. The restraint assembly was just sitting down over his shoulders, and none of the employees bothered to check, with that one guy too busy flirting with the girl. Somebody knew that something was wrong as soon as the ride began, yelling about checking the left side.
Seems to me that if there is any question about whether the safety restraints are engaged or properly inspected or checked, then that employee should have hit the kill switch immediately and brought it back down.
Big settlement forthcoming for that boy's family.