rhondagrensberg-smith - I wish there were an easy answer for you. Let me try and give you what I've got...
Autism is a neurological condition that causes the nervous system to be super sensitive to everything. Many doctors think that the filter that exists in the rest of us isn't functioning properly in a child with autism so literally everything gets through to them and they are overwhelmed.
Migraine is a neurologic condition that doctors currently think results from an overly active and sensitive central nervous system. Dr Marcus begins her post here https://migraine.com/blog/whats-new-in-migraine-research/ discussing this topic. In Migraine the central nervous system is overwhelmed by a physiological change (we call these triggers) and in those with the genetic capacity for Migraine a Migraine attack results.
While it is possible for a child with autism to also have Migraine, you can see that it might be quite hard to tell the difference between the effects of autism and the effects of Migraine.
A child with Migraine can be complicated to work with for even the best of headache specialists, but add autism into the mix and the case can seem daunting.
Typically, autistic kids like consistency. When consistency is gone, they can have a neurologic reaction that can be quite severe as their brains try to deal with the unexpected. Migraine will usually result in an inconsistent behavior.
I guess my question to you would begin with:
When you think he is having a Migraine, is he exhibiting behavior that is inconsistent with his usual behavior?
My relative with autism does not deal well with cold normally. If he started to seek out a cold place or wanting something cold touching him I would be concerned. Although many autistic kids thrive with body pressure which calms them, my relative also would not ever want pressure on his head, although he does hold his ears when sounds become overwhelming.
As you know, a Migraine attack is often extremely painful. Some autistic kids don't react at all to pain, while others overreact to it. Does he seem to be in pain?
If there is a history of Migraine in the family, it is prudent to rule out any inconsistencies that resemble Migraine.
If you know anything about Temple Grandin (herself a brilliant autist), you are aware that she likens autism to having an animal-like brain. As someone who has worked both in a veterinary setting as well as a training setting, we always are careful to assume that unusual behavior is coming from pain first. Rule out pain, then work to figure out what else it might be.
In my limited experience, inconsistencies in an autistic child's behavior should be ruled out as being caused by pain first. That is the only humane choice.
Can you get him to see a headache specialist who is also a pediatric neurologist? That would be my best suggestion to you.
Kudos and hugs to you for having your nephew's best interests at heart. Although it is normal for humans to use their own experience and project it on others, this is a part of our psychological makeup for a reason. It protects us and our loved ones, and it teaches us important life lessons. In our house we say "Plan for the worst, hope for the best."