My daughter has New Daily Chronic Headache (New Daily Persistent Headache / NDCH/ NDPH). I would try to get him CGRP injections (eg. Aimovig, Ajovy), as they are the only medication developed specifically for migraine and can work wonders. My daughter recently experienced her first 2 days of being pain free, after three years of 24/7 headaches and several migraine days every month.
All other medications prescribed to prevent migraine and treat chronic headache are antidepressants, beta-blockers or antiseizure meds. They do work for a lot of people of course, but if he has tried 3 different meds and they did not work, he can move on the CGRP injections.
I would read: "Pain in Children and Young Adults: The Journey Back to Normal" by Zeltzer and Zeltzer. It contains a wealth of helpful information. And "The Explain Pain Handbook: Protectometer" by Lorimer Moseley https://amzn.eu/d/1MnfbyF Watch: TEDxAdelaide - Lorimer Moseley - Why Things Hurt
Entertaining and informative.
Check out #themigrainelife on Instagram and take her course if you can. It will safe you a lot of time while you trying to get on top of this.
You could register for the Migraine World Summit on 25 March here: https://migraineworldsummit.com/ They offer a lot of free information.
I find the Manage my Pain app best to record pain levels and other symptoms.
I would not just read about headaches and chronic pain, but also about FND (functional neurological disorder). If your son has NDCH, he is organically fine. This is a functional problem. It is similar to a software problem in a computer. The hardware is fine. So regular blood tests are not needed if he has NDCH. They will not find anything, because this is a problem with dysregulation of the nervous system (central sensitisation).
The best book on FND is "Functional Somatic Symptoms in Children and Adolescents" by Kasia Kozlowska. It contains the most information and ideas of the ones I have read.
If you have questions, drop me a PM.