"Jejo" sounds as if you confused it with Russian её, I understand you meant "jej" - which would be 'her', 'hers'. Therefore you'd make it "He is eating her apple".
Basically, 'swój' and its forms are used when the pronoun goes back to the subject of the sentence - ergo, as you said, "his own apple". "On je jego jabłko" strongly suggests that he is eating some other guy's apple. "On je swoje jabłko" makes it absolutely clear that it is his own apple.