The CMs at Disney are told NOT to look at any paperwork, one reason is that they are not medical professionals and another reason is anyone can fake a doctors note, but the biggest reason is that they are not allowed to ask you for proof of a person's disability is that violates the Americans with Disabilities Act.
I notice you are from Quebec, it is possible the laws are different there and you need proof in your area. But here, you say he has a disability and no one can ask you to show proof, as long as the accommodations are to help him have equal access. (Accommodations to provide better access, such as handicapped spots in parking lots, they ARE allowed to get proof of that- that's why people have the handicapped placards on their cars. But that is only for cases of giving BETTER access. For equal access, no one can ask for proof.)
What you need to do at Disney, is explain his needs as they relate to waiting in line or navigating the parks (not just like regular life stuff like how it affects him at school, but how will it affect him being AT Disney?) you don't tell them his diagnosis, but just what NEEDS he has as they relate to waiting in lines etc
If you haven't done so already, I suggest you check out the first post of the "WDW Disability Access Service Card..." thread pinned near the top of this forum. That has all the basics.
As PP indicates, no note or documentation will be accepted. You will have to discuss his needs as related to standing in line. Not all people with autism require a DAS. The DAS is only used at attractions, not for dining, transportation, parades/shows, resorts, etc.