Ultimately it depends on how much he likes or tolerates his career field.
I have little shy of 8 years in now, so when I hit the 9.5 year mark that will be when the real decision is made. If you reenlist at that point, chances are you will so a full 20.
At this point, he can reenlist and totally change his mind at 10 years and be able to pursue another career entirely and still not be an old fart afterwards.
In my experience, the Air Force doesn't really play ball when it comes to giving incentives other than monetary bonuses (like choice of duty assignments, etc), so as far as I know all he could do was make sure his dream sheet says Germany, and perhaps talk to his career field's functional manager (detailer who deals with assignments, etc) and try to talk his way into Germany orders.
Of course if he likes his job but perhaps not the being in uniform part of it, depending on what he does he could always pursue a .gov civilian job or that of a contractor as well.