why does he doctor come? What does he do?

The doctor obviously wants Kino to reward him now that he has "the pearl of the world". Although Kino tells him that Coyotito is nearly well, the doctor claims that the scorpion sting has a curious effect that comes later and if he is not treated he may suffer blindness or a withered leg. Not sure whether or not the doctor is telling the truth, Kino nevertheless lets him see the baby. The doctor takes a bottle of white powder and a gelatin capsule, and gives Coyotito a pill.