Hi Daisy I live in Berlin but I am going on holiday now for 3 weeks. If there are any problems I am happy to try and help your son out when I get back (beginning of September).
The area that place is in is not really great, it's ok, central quite loud, not that close to his university. How can I describe the area, some nice cafes, quite a lot of Turkish immigrants, Potsdammer Platz in places - prosition strip. It's not Kensington but it isn't dangerous either.
Why have they not placed him in a Studentenwohnheim? THey are located all over Berlin, some nicer than others. Nicest I think are in Zehlendorf (Freie Universität - miles from the Humboldt), Schlachtensee (ditto south of BErlin, nice lake ,pain to get back to late at night) or Neuwestend (Sbahnhof Eichkampf, in the forest, used a lot by students at the TU but open to all). To be convenient for his course and a nice place to hang out in the evening, a Studentenwohnheim in Mitte/Prenzlauer Berg (where the Humboldt is or nearby) probably best.
He needs the housing officer at the Humboldt to find out which accommodation he has and where it is I should think.
I would write something like: Sehr geehrte Damen und Herren (of name if you know it), von (beginning date) bis (end date) werde ich an der Humboldt-Universität im Rahmen des Erasmus-Programs studieren. Man hat mir mitgeteilt, dass eine Unterkunft für mich bereitsteht. Ich bitte Sie darum, mir die Adresse und ggf. weitere Informationen dazu mitzuteilen.
Mit freundlichen Grüssen,
Joe Brown
Kind of thing, asking for the address and any more info they have on the accommodation. My mail is iskrennii at yahoo dot de if I can help him with anything when I get back