I see what you're trying to say. But the fact is, even if you don't tell people the child has Down Syndrome, it will be patently obvious on first sight. Unfortunately, there are those who will automatically assume the child is profoundly retarded...even if they interact with the child. That is something the child and the family will have to face, unless they cut themselves off from society. But fortunately, first impressions are overcome when they see the child's abilities. A child who's blind or deaf has limitations as well, and they also have conditions which can't be ignored.
Do you think the parents are making a mistake by telling people their son has Down Syndrome? I don't think they are making a mistake because these parents sound like they have every intention of encouraging this child to be everything he can be.