One morning, we were working on school. We took a brief pause so I could gather some materials for our next lesson. Grant grabbed a piece of paper and started writing his numbers. Then he moved onto writing musical notes. He showed it to me and said, "Look, it's music. Can you play it on the piano?" So I sat down at the piano and he explained where the first note was supposed to be on the piano. Then he found a toy skewer for his conductor's baton (from a wooden grilling set we have) and placed himself on the stairs and conducted me. After each playing, we'd pause so he could go add another note or two.
Later on, we got out some piano theory books and he really studied them then wrote some more music. He does NOT want me to teach him about music at this point. So for now, I'm just letting him play around with it and have fun. I don't know where his sudden interest in music came from but it's been fun to watch.