What is autism? How the term became too broad to have meaning any more

We have an autistic child and have faced backlash of a different sort, namely scorn from parents of "real" autists.

Autism is so difficult to pin down because it's a highly individualized disorder. Two people with a similar severity of the disability may exhibit completely different symptoms. Is one autistic and the other not? My child has an extremely difficult time understanding why or even that others feel things, and he will react in extreme ways to the slightest disruption of his schedule, and he has other ("mild") symptoms, yet he is able to attend school with "typical" peers (and lots of arm twisting, legal threats and an IEP) and by all outward appearances doesn't "act autistic." Is he not autistic?

Maybe we need two descriptions or to separate conditions by severity or something, but I'm not convinced we need to consider them as completely different and unrelated conditions.