I am interested in him trying public school. My main points are: 1) He doesn't want to be at the Montessori with the girls and younger boys which is understandable, 2) the public school has professionals that could help with the his LDs, 3) we'd start getting accommodations in place at an easier age so they'd grow with him rather than trying to figure it out in middle school when he might be resistant and implementation might be more difficult. I think we could after school the gifted side.
Just a few random thoughts -
Is this a situation that only has 2 solutions? I'm not sure if you're in the Seattle area, but wondering if you are since you tested with the Eides. If you are in a large metropolitan area, it's possible there may be more than this one private school choice vs your local public school option.
Next though - if you stay at the Montessori school, I'd work toward getting the accommodations he needs in place now rather than looking at the transition to public school as the place that accommodations come into play. It's great that his Montessori is providing scribing, but he also needs to work toward being able to write independently, so it's time for him to be keyboarding or using voice-to-text or whatever accommodations are appropriate. You most likely have a road map for accommodations as the years progress from his evaluation plus a sense of what will work from having parented him, so whether he's in public or private, get him moving forward with the accommodations he needs.
While I agree with your dh that the curriculum sounds wonderful at the Montessori, I'd also point out that there is more to a curriculum than what is studied. Project work as an individual isn't the same as project work in a classroom where there are additional students to group with (and where groups can change). Being able to discuss what is learned with a group of students rather than just the teacher opens up the world of considering other points of view. Having same-age peers during outside-of-school activities is great, but school is still the place that our children spend a huge proportion of their waking hours, so placing a child in a school where he's both the oldest student and also doesn't really want to be is something to really consider - is he going to be happy enough to really benefit from a great curriculum or will he be unhappy there?
Last thing, which I'd really seriously think about - why are his friends leaving? Why aren't there students attending in the upper grade levels at the school? Is there something else at play that maybe you haven't considered in keeping him there?
Sorry for all the questions - please know I'm not questioning your decision one way or the other... those are just thoughts that came to mind.
Good luck with your decision - it's not easy finding a good educational fit!
Best wishes,
polarbear