why is kino suspicious of the doctors treatment of coyotito?

He thinks the doctor is there because of the pearl. Even though Coyotito is extremely ill and at risk of death due to infection, Kino is still suspicious that the doctor has other intentions when he announces he'll be back to check on the boy.

Everything at this point is about the pearl. Kino suspects everyone wants to steal it; he trusts no one. When the doctor offers to lock the pearl up in his safe Kino refuses; he'd rather have in within close reach than trust it to ANYONE.