Also pronouns aren't necessarily dependent on gender, they're dependent on the person's pronoun preferences. Even someone who seems like they might use a specific set of pronouns based on their gender might use a different set of pronouns, as an example I knew someone who is a demiboy and they go by any pronouns.
It should be whatever Wakko prefers. Perfectly fine for a nonbinary person to use "he".
Edit: And also, pretty sure that's another layer to the joke. Cause Wakko is just out there living his life while the world spins around him making definitions and boxes that have no bearing on how he lives.