Berlin is a 4-5 hour train trip. I adore Berlin, but I wouldn't make that trip unless you can spend a week in each city. But, if you want to do Berlin, happy to offer suggestions.
Closer to Munich, Salzburg is only an hour away by train and tickets are stupid cheap. Regional train tickets anywhere in Bavarian are cheap. You can read more about that Bayern ticket deal here:
https://happytowander.com/bayernticket-guide/ (bonus: Salzburg is included in this deal). Nuremberg is about 90 minutes, or 60 if you pay extra for the faster ICE train. I personally like Bamberg, but that's because I love a good smoked beer.
What does your soon like besides soccer? Munich of course has the unfortunate distinction of being the birthplace of the Nazi party and the first concentration camp, Dachau, is just outside the city. So if history is of interest, Munich and Bavarian has it, although it is often ugly. The Nazi Documentation Center is an important and really well done museum.
The whole Olympic complex is neat, imo. You can do yours on the roof of Olympic Stadium. The BMW Headquarters and museum are nearby, if cars are of interest.
Since your son is 16, he's allowed to drink in Germany. That's obviously a decision to be made by your family, but I think part of the fun of Munich is experiencing the big six breweries. All have biergartens and hauses (Hofbrau seems to have one every other block, most of which are tourist traps, but there a a few gems), all welcome families and have delicious Bavarian food.