Both you and your partner have some points I can appreciate. Two thoughts:
1. You still have another year to discuss before middle school actually hits. (4th and 5th)
2. Regardless of your decision regarding placement for next year, I would suggest that you request an initial evaluation by the public school well before the planned transition, so that,
- you find out now what kind of eligibility and services they would offer, and what you will have to negotiate.
- the special education documentation is already in place, before he enters public.
Otherwise, you may find yourself slogging through the due process timelines for a good half year before he gets any accommodations. In many districts, the school will be reluctant to begin an eval on day one of school, when he's never been in a traditional classroom before. If they have RTI mandates, then they will want at least 8 weeks to take baseline and intervention data, before the special ed process starts. If you are in a state where you can circumvent RTI, due process still allows the district at least two months from your signature consenting to evaluation to complete the process.
If you start the process now, you should have a finding (eligible-IEP, eligible-504, not eligible) well in advance of his placement in public school. Even if you choose not to accept the offered services at this time, the finding of eligibility will remain on the record, and will likely make re-opening the process much easier. If you want your own evaluator's data to be included in the record, then you may either obtain private results first and submit them with the request for initial evaluation (the district will likely want to do its own supplementary testing, but at least they won't invalidate your private testing, since it will have been done first, and they'll know what tests have been given already, anyway; there is also a chance that they will simply accept your private testing data--though not necessarily its recommendations), or have the school testing done first, and then request an IEE if you don't feel comfortable with the results. The first option gives you more control, and gives your private psych first crack at the preferred tests. The second option costs less, and may prevent unnecessary duplicate testing (if you end up being satisfied with district testing, and not needing the IEE).
Asking for an initial eval, and then declining services at this time is not, btw, an unreasonable choice, as you have no way of knowing what the best educational option for your child is until you know what each one looks like. And best option will look different when he ages out of the Montessori. If you do go through this process, only to decline an IEP at this time, make sure to let public school personnel know that your plan is to return to the district when he runs out of grades at the private. It is also possible that the district will offer you related services, such as AT consult, or even before or after school OT, that are compatible with continuing in his private school, which would establish the record for special education while at the Montessori, making the special ed transition into public middle a bit smoother.
