My teen daughter suffered a concussion that caused numerous symptoms, but the most debilitating was headaches. She was not eating enough because of feeling so terrible. I did not realize this until she had lost almost 10 pounds. Once we became aware of this, we made a concerted effort to have her regain the weight. Under the doctor's guidance, she ate 2700-3000 calories each day for several weeks, and regained the weight. She is now back on her historic growth curve. After only 4-5 days of this high-calorie adventure, she stopped having headaches altogether and they have not returned. Her irritability, fatigue, and mental slowness have improved (not disappeared entirely). She is happier.
I just mention this in case you or your child have lost weight after the concussion. It is certainly not the answer to most post-concussion headaches, but it was a factor for us.