Good thing we didn't kill him in the womb or in the cradle then, as others on this page are advocating. His life was sadly short, but we are better for having had him.
To maybe phrase the other comment's concern more formally, the argument of whether or not to abort is filled with case-by-case circumstances, and saying that humanity is better for the contributions of a person without taking into account the extent of the person's or their parent's suffering is far too reductive to stand as an argument.
If someone had never been born, someone similar would likely have been born shortly later, perhaps without the same disease.
So your reasoning for treasuring his life over other possibilities doesn't hold.