LeChamois

I am not an expert in education but surely what you want to do about his weakness in German must depend on whether it is affecting other subjects or not. If he is good at Maths and everything else and was forced to repeat a year because of just one subject he would probably be bored to tears next year. As you have pointed out, he has only been speaking German for two years so he will probably steadily improve over the next two years. If he is not very good in other subjects either then that's a different story. I think you can just relax and take another look in six months time.