Autism / Autism Spectrum Support Group
Autism is classified as a neurodevelopmental disorder which manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior. Although the specific etiology of autism is unknown, many researchers suspect that autism results from genetically mediated vulnerabilities to environmental triggers.
Anyway, I'm sorry you are having to deal with this. You are on the right track with getting documentation of his appointments from his medical providers.
Also, you are right to follow up with the school and make them tell you why they have marked him as 'tardy' when they know why he is out of school. Definitely a good idea to keep records (phone log and notes) of your conversations with people (i,e, school, clinic) on this subject.
Good luck.