How old is your kid? If they are as high-functioning as you claim, then you should involve them in the decision-making about what to do. ("Do you want me to tell your teachers you have autism?", "Would you like plain food at school?", etc.)
When I was in school, the written "Individual Education Plan" said that I got an extra 30 minutes per hour on tests. In practice, this starts a conversation between the teachers that says, "this student needs special handling for tests". In practice, the accommodation I requested (and usually got) was permission to walk out of a test or exam as soon as I finished (sometimes half an hour early).
We haven't told him about either the ADHD or the ASD diagnosis. We will tell him the full story when he is older (both what he was diagnosed with, and why we are sceptical about the diagnoses.)
Since we haven't told the school about any of his diagnoses, he obviously doesn't get any special consideration for them. But he enjoys school, he reports no problems and neither does his teacher.
Thank god your son has a parent like you though — some medicate the personality out of their child, holding on to the shoddy diagnosis.
I hope as a society we can change all this — currently working on experimental assessments using direct brain data (eeg) to provide a bit of “truth” to the process.
 NICHQ Vanderbilt Assessment Scale—Informant