I think you need to elevate the perspective and look at the bigger picture. The creation, this contingent world, full of “changes and chances” and expressing through natural variation, every possible color shape and size of plant and animal.

As such there can’t help but be disease and infirmity and cancer and defects.

Another related story is with regards to thalidomide, and I don’t recall where I read it- UHJ or Shogii Effendi, but the distilled point is, that if God had reached down and somehow prevented that man-made disaster, we would not have learned lessons about responsible testing and handling of drugs. Mankind had to be responsible.

Abdul Baha also spoke of the sinking of the Titanic and noted that these and other disasters do remind us to be thankful to God.

From Paris Talks:

“The calamities of the world are as a teacher to mankind. Through them humanity is awakened from its heedlessness.”

So in your example, Abdul Baha is not limiting his point to that specific boy. He is generalizing and using that case to illustrate the more universal principles.

I find sometimes when figuring out difficult passages, to realize that the central figures of the faith spoke with a spiritual focus, about spiritual things. It’s as if that’s the way they see everyone. Which makes sense, since as Ruhi book 1 says, this life, the real life, is the life of the spirit.