As far as I know, it is possible to have the entry "ohne festen Wohnsitz"(without a permanent residence) instead of a mailing address in a German passport and he's legally not allowed to use his parents address, if he's not there for at least 183 days a year.
But I don't really understand how this small legal detail would change the whole character of his life experience, in any case. No matter what is written in his passport, he spends the whole year in a train.