Hm. He’s only 8 so may not understand subjects of the sentence and using the pronoun in reference TO him/her.
I would say time and grammar. I don’t know how else to get there. He’s still young, though, so keep going. (I know, not always easy.) It sounds like it took some time for the other concept to sink in and he’s also applying it at the other end.
Wish I could help more.
K