You can do it without a light... have him cup both hands over your eyes for a minute to minute and a half. Remove one, and see how the pupil reacts. Cover, and then remove the other one, and see how the pupil reacts. Cover that one back, wait at least 30 seconds, and remove both, and see if they react together.
I understand the timing thing, but head whomps are something you need to take care of. If you hit hard enough that you cried beyond what your self-control would normally permit, then you hit hard enough that you are possibly in a mild concussion. Concussions can be hazardous to immediately go to sleep with. Admittedly it is most with more severe ones, but it can happen that you don't wake up. And this isn't something you or KS want to deal with.
I'm a first responder. I'm not a doctor, or a nurse. But this is the standard I would instruct someone with a head whomp.
LthrOttr |