Good idea! The Instax gives instant feedback, like digital, but is more like a film camera. Hopefully, he will transition into traditional film cameras. Just be sure to process/have processed his films quickly so he doesn't get tired of waiting.
Gave one of my Grandsons a P&S 35mm camera. He shot one roll (24 exp. to keep it short) and I processed it quickly. Unfortunately, his focus wanders (not the camera, in his head) and it's now lost somewhere in his bedroom. Oh, well, He'll find it soon and hopefully try it again.