Translation:Το μυθιστόρημα του.
I am not an expert and I don't know if there is a grammar rule for it, but Greek is very phonetic language... you pronounce what you read... accents help specify which syllable gets slightly more stress. In this case two syllables in the word μυθιστόρημα should be pronounced with slightly higher stress. Paying extra attention on how I read the sentence (I am a Greek native speaker) I notice that Ι stress the syllable "στο" slightly more than the syllable "μα" and (of course) much less all the other syllables... don't ask me why...
Yes, it's a spelling rule as well.
(There also used to be a rule about an additional accent on the ultimate if the penultimate has a circumflex accent, but this was dropped -- perhaps because this was not reflected in pronunciation, since acute, grave, and circumflex accents have been pronounced identically for centuries and centuries now, and we don't write them separately anymore, either, since 1981 or so if I recall correctly.)