Autism Spectrum Disorder is a condition that affects many aspects of development. The development of language ability is affected, as is the understanding and quality of social interactions. Children who are on the spectrum will demonstrate behaviors, habits, or interests that tend to be rigid and repetitive. The may also have difficulty with sensory integration.
With the differences in the way that their bodies and brains process all types of information, it is also very common for those on the spectrum to deal with feelings of anxiety, panic, and/or repetitive or intrusive thoughts. From your description it does sound as if your son is experiencing some anxieties that may be related to school. Although his academic performance may be fine and his teacher does not feel that he is a disruption, he may lack the language to express these feelings, or may feel that he should keep them to himself. Speaking at a regular volume at school may be anxiety-provoking for him.
I would recommend that you consult with a psychologist who specializes in working with children who have developmental disabilities. Getting control of your son's school anxieties while he is young will help him to see school as a positive and supportive place as he ages.
Aja Temple, Ph.D.