I live in the western suburbs too! Have you investigated any of the gifted private schools in the area? There is Avery Coonley in Downers Grove, DaVinci Academy in Elgin and Quest in Palatine (but that's probably a bit far away to be practical).

From what you've said, it sounds like neither public school or Montessori is going to be a good fit for your child. I don't know alot about these schools, but they may be a reasonable option to consider for your ds if you can afford the tuition.