"We are eating his bananas."
Translation:Wir essen seine Bananen.
We are talking about someone having/eating something, so we are using the accusitive case. It is essential to unserstand the cases, there are four of them. I found it really hard, got stuck here for about a month. But, once you get it much of this makes sense. For now, to answer your question, when the piece of fruit is owned/eaten by the subject, the sentence is nominative, then a plural piece of fruit will always be seinE. The E is the sign of it referring to a plural, or a female thing. A male object will be seinen, and a neutral object will be sein.
Seine Bananen because Bananen is plural- therefore it makes the "e" on seine
Seinen Orangensaft because der Orangensaft is Masculine- so in Akkusativ it becomes Den orangensaft. Since it is "his organgejuice and not the orange juice" , we do the sein version of den which is seinen. Seinen Orangensaft