Quick question - what do others do for language learning? I'm planning a short jaunt to France and maybe to Austria, as Liebs has some French and says he would like to learn German now, but how do I start him off? My German isn't that good so it will help me too.
Depends what your son enjoys doing really and how motivated he is to learn. You could start with introducing food or music or art, set a cultural tone and atmosphere. There's books or you could get one of those computer programs if you think he will like that. Or just browse the web, watch YouTube tutorials and that sort of thing. Or attend a class together? You could go around the house together labeling things with sticky notes. You could watch shows with english subtitles on. Play memory matching the other language word to an English word or picture. All sorts of things.

You don't need to think of how to introduce something in an unschooling way, any path to learning can be unschooling if it is travelled by choice. Just offer ideas and resources without expectation and support him through the ones that resonate best.