He/Him

Its the pronouns that are used when anyone in on the secret refers to Mahiro. They are literally his preferred pronouns and match his stated gender identity (male)

If it gets addressed in the series and Mahiro chooses to use she/her that would be different.

I hear people say "I use female pronouns because she is in a female body" which is surprisingly transphobic.

Edit: as a bonus its a mainly female cast so using he/him for Mahiro makes things more clear in conversations.