As a postdoc, how to deal with supervisor who keeps trying to involve my student in other projects?
This is a tough one.  The German academic system is more of a pyramid than in the U.S. Too often, the guy at the top of the pyramid got there by, shall we say, not being the politest, most patient piglet in the litter.  So I understand your reluctance to assert yourself.

I also understand your resentment of the boss setting up distractions for your student.  You have described a pattern of subtle sabotage.  The student should be your responsibility.  From your point of view (and mine), the boss has no right to butt in.  From his point of view, he has every right, because he is the great man, who has earned a high position of great respect, and has years of wisdom and good judgment to draw on.  Since you are the student's advisor, and the boss is your advisor, the boss concludes that he has the ultimate responsibility for the student.

I think the dilemma you are in is inherent to the hierarchical structure of German academia.  I've never heard of a situation like what you described, in the U.S.  In the U.S., which is the academic system I am most familiar with, post-docs don't advise PhD students.  But, from what I know of the German system, my guess is that it is not extremely common, but not rare either.  Is that about right?

I hope you can get a different job soon....  In the meantime, the best that I can offer is empathy.  Possibly you could talk this over with the department chair, or with the equivalent of the dean of the graduate program?  Or could you say to the boss, "Frank (I made that name up) is very interested in such-and-so project that you have proposed for him, but he can only spend x % of his time on it (e.g. 10%), because his primary responsibility is his principal project."  In other words, start with an assertive stance, but be prepared to negotiate somewhat on the percentage.

I wonder if there are any gender aspects of this?  If so, is there an office at your institute that supports women in tricky gender situations?

Have you talked things over with the student?  Perhaps he feels some divided loyalties, and finds it difficult not to defer to the big name?