Id make an appointment now for a WISC 4 the day he turns 6. You don't need any test right now though.
Have you sat down with the teacher? If so...what happened?
Testing is great if you aren't sure but with your childs milestones it is not really a question of if only of how unusually so. Teachers usually aren't interested in test results.
What grade does you sons school go up to? What grade level material does the school stock? If they don't go past 2nd grade you may be waisting your efforts.
Montessori has a lot of strengths but the typical weakness is that a teacher who isn't herself quite bright will tend to dogmatize the developmental timetables and only see the bits of your child which match the preconceptions. I see this as the risk any group of people with a quite good model of reality run. Myself included...if my only tool is a hammer then everything starts looking like a nail. That's human nature.
A good place to start is an hours worth of silent observation at the back of the classroom on the pretext that you are concerned with your sons behavior. The you can meet and share your observations.
Id also recommend reading Ruf's book - 5 levels of Giftedness.
Smiles
Grinity
