Unfortunately you are going to be coming up against this throughout his life as there are many people like your mil who refuses to lean about special needs.
I can relate first hand, the worst was a family member who insisted my full time wheelchair using DD walk around a museum because she was just being lazy and I wasn't giving her the opportunity to try it !
There's been more incidents with many other people but what you should do is come up with a sentence that explains your child's needs and that they won't be doing it, you firmly let them know that this won't be happening,
You are his mother you know him best
It really is unfortunate that there are people like this, we end up keeping to ourselves because it's easier than explaining to someone. Just when we think our circle is well in the know suddenly someone pops up with a ridiculous suggestion, I only had it again and it was dds teacher who works in a disabled school organise this year's school trip somewhere totally unsuitable and here I was yet again explaining, I try to remain calm but those old angry feelings cropped up again and I just want to shake her.
Come up with a plan to say to everyone each time they suggest something or somewhere, be firm but polite and never back down to something you are uncomfortable with. You are your ds voice, he depends on you keeping him safe and he trusts you, don't break that trust, he knows you won't make him do something he can't.