First of all, I am so sorry that you and your DS are having to go through this. Starting middle school is tough under the best of circumstances, let alone with his profile--and then this situation on top of it all.
A few things stand out to me:
1. How has he not been reevaluated since 1st grade? Reevaluations are mandated every three years for students on IEPs. If this is the case, they are way out of compliance, which puts you in a much stronger position to get compensatory services and/or reimbursement for alternate placements. Don't forget to request OT (fine motor/handwriting/sensory) and assistive tech as part of the comprehensive eval.
2.If they are having to use disciplinary measures on a routine basis, and have already called you in for four IEP meetings, they should have been the ones requesting consent for an FBA. More leverage for you in a potential hearing. Don't sign anything you don't feel comfortable with. And document any incident he describes to you that sounds like it might be restraint or seclusion.
Okay. Now for your actual question. I would say that the WISC is probably still your best option as a cognitive assessment instrument. I understand that you are concerned about ceiling effects, but remember that the norms go up to age 16-11, so although he won't be able to generate the same range of extended index scores, there should still be plenty of ceiling to generate 18s and 19s. Even 16 year-olds can earn 18s and 19s on nearly every subtest.
