I wouldn't worry about language difficulties too much, your DS will pick up enough German to deal with school by the time he is 6.
I moved from the UK to Germany with my British parents when I was 7, to a smallish town near Stuttgart, and started school there halfway though the first year of the Grundschule. I did almost all my education in Germany, apart from almost 2 years at a British school in the UK when I was about 13 or 14.
For us it was a very positive experience. OK, so we struggled a bit with the language at first (not really a problem for me because I was young, but it did mean that my older brother, who moved when he was 9, had to repeat one year of Grundschule because he was sent to a special class to learn some German when we first arrived), but that sorted itself out in time.
My parents spoke no German when we arrived, and never helped us with homework, except sometimes with our French homework when we got to the Gymnasium, but it never really mattered - I felt that overall our teachers were quite supportive, and helped us as much as they could. Also, my parents had a very relaxed but supportive attitude towards marks and tests, as long as we just about got by, so I never felt stressed by getting bad marks at times.
I am very glad I went to school in Germany, but then I am very biased because I absolutely hated the two years I spent at a secondary school in the UK - I found it cold, uptight, full of rules, (uniforms and queuing in nice rows outside classrooms are not done in most German schools!), relations with the teachers were overall not that great, and there was lots of bullying and nastiness that I never encountered at any of my German schools, and I found all the hanging around going to assembly and stuff really boring. My parents also preferred the German system once they had got used to it. I found school relatively easy, so I guess it wouldn't ultimately have mattered where I was, but my brother has never been very academic and struggled in some subjects, but he still ended up with a good secondary education, was very happy at school, and got into a decent university.
Re. homework: I think this will depend more on the individual school than on the system. Some of my friends in the UK tell me their children get loads of homework and spelling tests every week, and some tell me they get nothing, same goes for my friends in Germany. A lot of my friends in Germany are working mums and they send their children to a Hort (a sort of afterschool), and the children do their homework there, so that could also be a solution for you if you are worried about not being to help with the homework because of language difficulties.
I think the main disadvantages about the German system are that:
- It seems idiotic to send children to different schools at the age of 10, it disadvantages late developers, and changing schools is not totally straightforward (even though I know loads of people who did it). There used to be some comprehensive schools in BW, but I think they could have been been changed status now, which would be a shame.
- it is probably true that there is not that much support for kids who are struggling, but that was not our experience, my mother always felt that my brother got about the same amount of support at his German school as he did in the UK before we left for Germany, but that could have changed over the years, and the UK seems to have done a lot to support struggling pupils and make sure they don't fall behind. They probably do have a lot of tests of Germany, which could cause stress for some, but I think that any education system will have an element of that.
Anyway, good luck with your decision, it is not easy to make decisions like this. I think about it quite a bit - my children are in the Romanian system (my husband is Romanian), which I also find very far from ideal!