...are you thinking being Vegan has something to do with this? I can assure you it's not. Other than food allergies that trigger ADHD/Autism-like symptoms, and usually it's good old fashioned processed grains like wheat and corn that does that...though there's a few that will disagree with me on that.
My son is on the spectrum(evaluated through SELF, and attended there before going into his regular school). His school has a lovely blend of children, a good portion of which went through the SELF school, not that all of them needed it, but going there and being EVALUATED helped the parents narrow down things that needed looking into. Medicating children? SELF won't even suggest it. That's for professionals. They evaluate what can be taken care of with EDUCATION, and guide the parents to look at medical professionals if this isn't enough. "Choosing a path" isn't that difficult with the right information, which is with evaluation, not labeling.
The hitting his head for asking to do anything? Attention. My friends then four year old did that because she had a three year old sister getting more attention.
She quit when I told her that the only thing grabbing attention like that was getting her was an extra helping of (insert food of the week she didn't like), and a headache, followed by nap/bed without dessert. She quit rather quickly, as she was fond of her pudding.
Mind you he doesn't have to be a sibling for this to occur. Anything could be taking attention from him(Phone, TV, MamaPedia, even the pets). At that age, they NEED us and only US...until they make friends at school or on the playground, WE are their #1 bestest pals in the whole world. They need 1,000% of the attention.
How recent has this head banging issue gone on? Other than a non-important factor(I don't see dietary preference as important versus things around him happening), and some head-banging, we don't have much information.
Take it with a grain of salt, as this is all...
Just my 2cp.