I think you need to find out for certain if he needs to be enrolled in a school orchestra to access the regional/festival orchestra activities. I would ask the school teacher directly, or try and figure out how to contact one of the festival organizers- I suspect these things vary a lot by region/location. You might try asking through a neighboring district if they have a stronger program.
It sounds like the middle school orchestra isn't going to offer him much; the problems with fooling around and not attending to the task at hand sound to me like problems with how the teacher is running things (is it the same teacher for all grades/years?). The thing about first and second violin parts for different years is just bizarre.
If he needs/elects to stay in the school group, I would see if his private teacher can communicate intermittently with the school teacher. Ours does, fairly often, to make sure they are on the same page with solo choices, timing of certain things, outside opportunities, etc, but the private teacher also helped with suggestions for differentiation in school. He is the one who pushed for more, and more varied, small group ensembles- this won't help if your DS is the only advanced player, but perhaps they could help team him up with a few high school players? The small groups here don't usually perform in concerts (there are too many) but they do participate in festivals and judged performances.
In our high school, the kids who take private lessons spend their school lesson working on small ensemble work. (What does your DS do during school lessons? Can the private teacher help with this so he doesn't have to waste that time sitting in on an inappropriate lesson?). In middle school here, some lessons are group (but they are grouped by ability) and some end up being essentially private if the student doesn't have near peers. It takes a lot of dedication on the part of the music teachers to make this work- they are almost always giving lessons before and after school in addition to the full rotation during school.
As far as making school orchestra tolerable, a lot will depend on the teacher, I think. Here, advanced kids are sometimes given solo pieces, either accompanied by the group, or as a featured part of a regular orchestral piece. (There are lots of advanced kids, so it doesn't seem that strange to the kids to have someone singled out, many of them are throughout the year). Sometimes the band director chooses music that features a strong section, that helps. And I often remind our kids that they can always improve their own playing, even if the music is easy. Work on the vibrato, focus on breath control, really work the dynamics, etc- not a solution, but it helps.
