I lost my little boy in a late missed miscarriage.
I already had a small child but their father left before this happened.
The amount of people who tried to sweep it away by saying it was probably for the best because my ex wasn't with us anymore...
It doesn't matter what reason they give, no one should ever say it's probably for the best. These babies are our babies. Our children. Real humans.
Your son was your son, whether he had DS or not. I would be inclined, if you're not strong enough to say it, to type down that its distressing and inappropriate and please refrain from saying it, and printing it out to hand to people.
Nothing wrong with people knowing, they might even think twice about saying it to someone else.
I am so, so sorry for your loss. I've lost three, I have two and can no longer conceive, but I've finally come to terms with all of that, it's been several years now. Allow yourself time. And you don't have to speak with or see anyone you don't want to right now.