You don’t need to have a formally recognized “official” fallacy name, to refute an argument based on false assertions. All you need to do, is point out that the person isn’t using any accepted facts, in order to draw their conclusions.
As for the subject of Autism itself, that is a tricky thing to discuss anyway, because it is one of many human problems which cannot be determined biologically. There have been no physicalities of any kind associated with it (one of the FACTS that your opponent ignored/falsified). It is currently arrived at as a diagnosis, by observing and interviewing possible sufferers, checking for as many alternate explanations as possible, and then applying the label.
It is currently a SYNDROME. Not an formally recognized Disease.
Disease itself, is one of a number of words, which is used both as a vague general term (what might be called the “street definition”), and as a formal Medical Science designation. In casual conversation, “disease” can mean “anything that bothers you.” In formal medical parlance, it refers to a medical problem with a specific known cause.
AIDS became HIV, after the SYNDROME (collection of symptoms) was proven to be caused by a specific VIRUS. SYNDROME is the S in AIDS. VIRUS is the V in HIV.
Autism is currently a SYNDROME, not a Disease. It may be, that some day, one or more physical causes will be discovered to explain it, and allow direct solutions to be applied.
Until then, anyone claiming that an autistic person can be recognized from physical features, is either purposely lying, or is simply so ignorant that their arguments based on their ignorance can be declared invalid on their face.