An extra -s or -es is added to masculine and neuter nouns in the genitive.
-es is often added to nouns with only one syllable: sein Buch – der Titel seines Buches/der Titel seines Buchs
-es is always added to nouns ending with -s, -ß, x, – z, -tz: mein Glas – die Farbe meines Glases
-s is added to nouns with more than one syllable: unser Hotel – der Name unseres Hotels