Oh boy finks this doesn't sound good, after all the worry with ds1. I have a lot of thoughts buzzing around in my head about the school system generally and all of our different circumstances as well but can't put them in coherent form yet.
How do working mums manage? They don't. It's a well-known fact that their dc are over-represented in Haupt- and Realschulen, underrepresented in Gymnasia. Media are always bemoaning the fact that for the most part the dc of well-educated SAHMs are the highest achievers in the German school system. If the mum isn't compensating for school deficits at home, you can forget it.
And I agree, IME raising concerns/making complaints to the school achieves nothing. Save your breath. If anything, I'd go straight to the Schulamt these days and I've only ever heard failure stories about that path too.
Finks, I'm a bit confused about ds2's sitution. You say he has problems with the language barrier but I'm wonderign why this is so as a half-German boy who grew up in a small German village, I should have thought his German would be equivalent to that of his classmates.
What came to my mind was getting an older boy to tutor him, like a kind of big brother. Would there be a nice lad from sailing club or families who is doing well at school himself that you could imagine roping in for that?
About the Gesamtschulen, I thought they had to give applying siblings preference. Isn't that so? I would cover my bets and apply to the Gesamtschule further away and fight to get him in there despite the zoning if the other one doesn't work out. As you said the head was keen. Could dh write to the Schulamt, playing the language card and see if an exception could be made?
What kind of test was he doing in German? Explain in other words what "Gelehrte" means or write a sentence using the word or what? Am trying to imagine the situation really so we can see if we can find a good solution for tackling future tests of that type.
Confidence: how about getting him to help/teach other dc in some role? He's only young himelf I know but if he can have some responsibility for guiding/instructing younger dc taht would give his confidence an enormous boost. I'm thinking cubs/sailing/English - can you see anything? Drama is always supposed to be good, if it is on offer and you can imagine him doing it in any way?
How about teaching him Latin? I bought the Minimus Latin series which finally arrived yesterday. Looks very good, comics, history, myths, grammar nicely presented. Learn all about grammar and translation which is a great tool for dealing with other languages and I see they have Roman days at the British museum in London and weekend sleep-overs and all sorts of great ideas for which dd is still too young (from 9 I think) but can make a real hobby of it and keep the will to learn alive and them all fired up about something. They say a minimum of half an hour a week would do.