Hi oranges, OK, now with my detective hat on I know which school you must be talking about (not too difficult, there are only two junior/senior school combos in Berlin, and only one of those has 8 to 4 lessons!). Congrats, it's not easy getting a place there nowadays! Do you count as 'highly mobile' or were you just immensely lucky?
I didn't know that school has maths in both languages for the first two years, that's an interesting policy. I think it's because the school has so many 'highly mobile' families so there are a higher percentage of English-only families entering the school (whereas the Europe schools for instance have a majority of bilingual children).
Actually you're in a really good position there. The secondary school is a Gesamtschule, which means there is less pressure in the 6th grade to get that magical 'Gymnasialempfehlung'. OK, I think the sec. school does practise streaming, but it's far more flexible than being pushed off into a Sekundarschule and having to work your way back into the Abitur system (quite apart from anything else, there is only one Sekundarschule in Berlin with a bilingual class, and last year it had 7 pupils!).
The teachers at your school DO have a fair bit of experience in knowing which kids are struggling with the German-language components, and if they recommend repeating, TBH I would tend to trust them. It's unlikely your DS will be bored, because he will have to put so much extra mental work into learning German and doing maths in a foreign language that a few dreamy hours in English and Topic won't harm him at all.
What really puzzles me - I mentioned upthread about the way that ages are calculated in Berlin schools. If I've calculated correctly, the correct year group for grade 3 should be born January to December 2005. There is a slight complication with 'Kannkinder' and 'Musskinder' - for a child born between January and March each year, the parents can decide if the child should be in moved up to the year before or stay in their proper year group (ie a Jan to March 2006 child would have the chance to be in the 3rd grade, they are Kannkinder because they 'can' be in wither year group) - but this option is not open to a child born in April 2006 (they 'must' be in a particular year group). So actually, your DS really should be in grade 2 after all, according to the Berlin rules (and the age in Berlin really is younger than average for Germany). This might be why you find the others in the class older - because they are! One way you can find out: we generally have a class contact list prepared by the parent reps and sent around by e-mail. Often the children's DOB is given on that list so you can see how old the others in the class are. Or send a mail to your class rep. If your DS is among the youngest, I would certainly consider him repeating the 2nd grade.
Sorry for all the waffle - feel free to PM me if you want any more gossip info. And welcome to Berlin!