Translation:The boy eats brown rice in August and September.
I'm guessing that for the rest of the year han spiser æbler - everyone else seems to...
Looks like the word is used in plural (same form as singular: ris, -en, -, -risene) when talking about rice as a "collection of rice grains", i.e. usually when talking about rice in recipes or eating rice.
Duolingo says "En rød bil (a red car) becomes den røde bil (the red car). In this case the adjective is declined the same way as for the plural, no matter the grammatical number or gender of the noun.