Tell them to contact the National Autistic Society and let them know they've found a revolutionary new treatment for autistic behaviour and would like to share their acumen and insight. (sarcasm emoticon)
What absolute fools they are and bullies too. Most people can be educated about autism, some unfortunately are too thick, stupid and pathetic to even take on board simple parenting strategies for autistic children.
Cut their contact with your son, he doesn't deserve to be treated like that and neither do you. He's 11, he needs support, how's school, are they helping or do the other kids have a go at him in the same way your husband and ILs do? The exact words you used were used by other kids to my dd when she was in primary school. It's bullying.
Their ignorant, denialist, thick as two short planks attitude is unfortunately a very common occurrence in families, friendship groups and even worse, some professionals. 
The only way you are going to make them see how autism affects your son and to understand that autistic people are "wired differently" is for someone in authority to explain it to them. That would probably have to be a man 