substantival possessive pronouns     German English     feminine masculine neutral     Nominative seine seiner seines his   Genitive not used * not used * not used * of his   Dative seiner seinem seinem to his   Accusative seine seinen seines his               Nominative seine seine seine his   Genitive not used not used not used of his   Dative seinen seinen seinen to his   Accusative seine seine seine his

* In common language the Genitive of the substantival possessive pronoun is not used. There is another form of a possessive that could be used (der seinige, die seinige, das seinige), but it sounds really antiquated. If you want to form a Genitive of seine it is mostly done like in English.

Das ist der Schlüssel von seinem (Auto etc.). This is the key of his one (car etc.).

The same could be said with a real Genitive and seinige (it's just not used commonly):

Das ist der Schlüssel des seinigen. This is the key of his (one)..

Example: die Schachtel (= box) possessed object => feminine, singular

Ich sehe meine, aber seine nicht. (Accusative) I see mine, but not his (one).

Example: das Kleid (= dress) possessed object => neutral, singular

Du kannst mit seinem hingehen. (Dative) You can go with his (one).

Example: die Häuser (= house) possessed object => neutral, plural

Ich sehe meine, aber seine nicht. (Accusative) I see mine but not his (ones).