Is corn and or corn syrup ok for an autistic child

I'm a Gramma of an autistic grandson. He has been on a gluten free diet and as well drinks goats milk. He tolerates goat products fine.

My sister found out today through allergy testing that she cannot have gluten, but was also told to tell me not to give my grandson any corn or any type of corn syrup, or any sugar. Is this true ?

He presently eats the Enjoy Life Very Berry snack bars, chocolate chip cookies/gluten, casein free by Pamala and loves blueberries, pineapple, banannas, most any fruit.