I may be late in giving you an answer since your post is a little older but I do have the perfect school for your boyfriend. It's called the Frankfurt International School and it is located in Oberursel. They teach most classes in English (some in German). It is a very expensive, private elementary and high school. They are always looking for teacher assistants, substitutes or even full time teachers. He should visit their web page.
there are international schools in all large german cities (e. g. hamburg, dusseldorf, frankfurt, stuttgart, munich, nuremberg, berlin). worth trying. look them up on the internet!