I think he'd have been better to use the term "lucky" but actually I don't really see that what he said was that bad. I have come across a heck of a lot worse from people who have foot in mouth.
You don't see it as that bad because the othering of people with a disability is so ingrained into society.
Ask yourself the following, which of these would you not have an issue with being said, or saying yourself:
I'm blessed/lucky not to have black children.
I'm blessed/lucky not to have homosexual children.
I'm blessed/lucky not to have female children.
I'm blessed/lucky not to have children with disabilities.
I bet (or rather hope) you wouldn't dream of saying the first three but apparently there are no qualms with the fourth. Why?
A child with a disability doesn't make a choice about it, just like they don't chose their race, sexuality, or the sex they are born as (please for the love of the donkey don't jump on that as a signal to start the trans debate here)