A repertoire is not built overnight, and for most players, having a deep theoretical knowledge is not required. They simply need to understand opening pawn structures.
That said, if this child is showing a great deal of aptitude, it may be worth adding this to your study regime earlier than I would normally recommend. I have seen two prodigies up close, and I will share what I have seen: I have known GM Josh Friedel since he was 6 or 7, and his first coach, who was the first player that I REALLY wanted to beat, has been a close friend for 40 years, so I know that he taught Josh openings even at a very young age.
My second encounter with a prodigy is the amazing 8-year-old Ryan Sun, who is now already rated above 2000 (and plays stronger than that). I faced him in my first game back after 16 years, and again, just this past weekend. I went 0-2, but in our game this weekend, it was especially clear that he was well-versed in the opening. So, for certain kids, who are gaining strength at an incredible pace, they do need a repertoire early since they will soon be facing players, who are booked up.
Unless you are an expert at opening play, you need to do one of two things at a certain point: Pass him off to a stronger coach. In Josh's case, he "graduated" from my friend to a GM coach, I think at around age 11. The other option is to buy repertoire books, that were written by grandmasters.
For kids, who calculate like little computers, you probably want books that focus on aggressive repertoires, typically 1.e4 as white, the Sicilian Defense as black versus 1.e4, and then a GM-suggested repertoire versus 1.d4. There are a lot of such repertoire books out there.
I hope this helps.
Below, the original poster asks about how to prepare to present the material, and how to prepare to present it, and I added a comment. I will that here too.
Usually, those books are pretty complete repertoires, and you need to go through a good portion of the book to make sure you cover everything. So, take each chapter, and they do not have to be in order (for example, if the first chapter is oddball sidelines, that is not the place to start), and read it yourself, and get a feel for the plans and ideas; and then start presenting them chapter by chapter until you are done with the book. He will not have to memorize everything, but this way, he will be exposed to all of it. Just keep building on that.