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)..