Maybe a better topic for the bar.. but .. I was barely able to communicate in English after school. When starting to have friends at university from China, Greece, Italy .. I quickly learned the basics, and not being affraid to stumble since no one had perfect expression. Just make things work. Then watching StarTrek DS9 and other series in English with subtitles. Subtitles in the foreign language! So you have some text to read if the prononciation is too demanding at times. Movies/Series in original are so much richer than in translation.. and movies have a very good translation to German, but they are .. bland. This is a fun and effective way of learning. One episode per day. Best if you like it and have seen it in your mother tong before. So you have kind of an idea whats going on even if you dont quite get every word. StarTrek TNG is from all the series especially good because they have a clear and slang-free language. Starting with Battlestar Galactica maybe not the best idea in this context
Books also help.. I started with reading Harry Potter in English.. every other book since then also in original. Some are harder than others. Iain M. Banks is perefct in English for me (German translation sucks) but I still cant read Terry Pratchett, I just dont have enough background to appreciate the jokes for instance. You have to be interested in this of course. If you do it because you must, its not gonna work. So try to find a series you watch with your son you both like. In German, with German subtitles. Then make it a habit for dinner or after. Good luck!