the Rhine River cruise (use KD Weisse Flotte) and Heidelberg with the castle lit up at night as viewed from the "old bridge". I would also include all 3 of Crazy Ludwig's castles--Neuschwanstein (his dad's castle is very close to this and you could stay overnight in Hotel Mueller which is at the base of his dad's while being able to look at Neuschwanstein), Linderhof (with the grotto under repair, it isn't quite as powerful as otherwise, but still great), and Herrenchiemsee, his Versailles (saves a trip to France!)--all of which are very reachable from either Munich or Salzburg. At the same time (same general area) you should also consider a Kehlstein (Eagle's Nest/Hitler) tour, Sound of Music tour, and the Salt Mines tour.
A great company and a great site to check out ideas is: to-europe.com
I am taking another group of 8 in June for 15 days and only visiting Germany and Austria, and will only scratch the surface of all that is there. Your son's German will probably aid you in having an even more enjoyable experience, and he will find it even more enjoyable too! Viel Spass!!