I’m glad for him and for his parents; America to the rescue again. :) I hope he lives a long and happy life.
I do have some reservations, though, about the precedent this will set. Why wouldn’t a temporary visa work? Or some sort of guest status?
Perhaps the treatment takes longer than some sort of visa would extend? I remember having to go to see authorities in Canada to extend my student visa after I met my husband. :)
There are medical visa’s available (Nonimmigrant Visa Electronic Application DS-160 form).