Can you insist that he eat a small amount of something you choose before he gets the teddy Grahams/goldfish/whatever?
It can be very, very small at first...just to establish the habit. Like, one piece of corn.
Does he take a multi-vitamin?
You listed a lot of crunchy foods. Maybe tried veggie chips or freeze-dried fruits and vegetables? Will he eat anything with protein?
Can he have soy milk during the day instead? Also try almond milk or coconut milk.
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So you know who just said that:
I am female, I am married
I have two children (one AS and one NT)
I have been diagnosed with Aspergers and MERLD
I have significant chronic medical conditions as well