From my experience pre- and post surgery, Chiari has affected my personality and ability to control my temper. It has greatly improved as I sit at the two year mark. Time, but also hard work in a number of areas have both helped.
In discussion with a neuro-psychologist, my family doctor, and other medical folk, my understanding is that CHiari can manifest itself like repeat concussion in terms of anger and explosiveness in "stressful" situations (think of foot ball player who start getting into fights with the people around them). I put brackets around "stressful" because the situation might not seem stressful to others. I do not know how old your son is but please do not mark it off as this just being a phase.
If you talk to parents whose children have experienced concussions, some will have the same experience as you.
A brain injury clinic in your area should have the resources to help you. Sometimes they can question the mechanism of injury, but again, my understanding is the CHiari can effectively damage the brain - think decreased blood flow to areas of the brain and that turns into damage - just in slow motion, not with the speed of a car accident.
In the mean time, read up on what kids with concussions should be doing. It is more like not doing. Minimal screen time, lots of rest (really rest, not "I'll just check my emails), decrease noise levels (machines and people), declutter the house, quiet meal times, one person talking at a time. All the stimuli that surrounds us every day is hard for the brain to process.
I know that my neurosurgeon said that I would be ready to go after 3 months.
He lied. (and young people do not always bounce back)
I had to greatly review my expectations of my abilities and what was realistic. After lots of practice with watching my energy levels and environmental stimuli, I am getting to be a better judge of what I can do. Please know that when I do too much, I irrationally yell at my kids. I do not feel particularly proud of that. My brain stops functioning properly and when a situation gets too much (in reality is a minor set back that requires me to be flexible in my thinking) my brain shorts out.
Your son needs you to help regulate his environment and to understand and to help him with getting better. He does not like being out of control either.
Well, give what I had to say a thought. If it jives with what you are thinking I have more to say!