We live in Austria: we all talk mainly English at home and the dcs (8 and 4) speak German at school and kindergarten. (DH is Austrian and speaks fluent English, I am English and speak fluent German.)
Our 8yo ds is learning to read and write in German at school - he's struggled with reading, though it's coming on quite well now. As he was having a hard time, I didn't want to confuse him with doing reading and writing with him in English too, though he does seem intuitively to read well in English (shorter words, I guess, and probably after all those years of bedtime stories - a bit of osmosis!).
As he gets more confident with reading and writing in German, I'd like to help him with English; we're considering a bilingual secondary school for him, and I think they request that the children can write in English.
Any tips for me? Any particular workbooks we can do together? Or particular learning tools? I had a quick look on Amazon but nothing leapt out.
If he is intuitively reading OK in English as you say, then don't bother with learning schemes and what have you- just get him reading. The writing will then come.
Dd is bilingual Italian English and we have the same language set up at home/school that you describe. Almost all her "story" books are in English, (simply because English children's literature/publishing/Amazon is so bloody user friendly ) and although she does make some spelling mistakes in writing, it's coming along and she will sit happily writing stories/emails etc in English.
I think you are totally right not to push the English until your son is totally to grips with German, that's what I did (dd is now 10 btw)
Maybe enrol him on some gamey type English language websites? Harry Potter? Pottermore? National Geographic? These are all sites dd uses in English, which obv gives an added impetus to use the language.