Have you feedback this development to the pediatrician? As someone who had fell on the back of my head before, there tends to be some medium-term side effects even though imaging showed no brain injury.

How old is he btw? One thing I learned is to record down the time the symptom was noticed & present it to the pediatrician so that they have a better idea on what took place.

Frankly speaking, I feel that kids at this age will usually laugh if they think what they're doing is silly / funny. Therefore, in my opinion, I don't think he's fooling around. The only thing we can't tell is if it's something to do with the concussion or the MRI contrast.