Getting ready to sing?

Isaac doesn't sing at all. He hums sometimes, but it really isn't musical -- it's more like vocalizing. However, he likes to listen to us sing, and he's fascinated by musical instruments, so I figure he'll sing along eventually. Perhaps these developments from the past week indicate that he's getting ready to do it:

1) Twice a day, at naptime and bedtime, I sing a lyrically revised version of Skip to My Lou as part of Izzy's going-to-sleep routine. I've been doing it for at least a year, but he had never shown signs of noticing the song (unlike Young MacIsaac) until last week. As usual I introduced the song by saying, "And now we sing," but before I could start singing, he said, "Lou Lou." There was no tune -- he simply spoke the words -- but I was excited to discover that he knows his bedtime ritual so well. (Why do I still get excited? I should know by now that he pays attention to everything.)

2) Our weekly Kindergym class ends with everyone gathering in a circle to sing. A few of the other little kids sing along, and many do the hand gestures that accompany the songs. Isaac just sits and listens, but it turns out that he is paying attention to the songs we sing (see #1 for my inexplicable excitement at this). At class last Thursday, instead of sitting on my lap as usual, he sat down in the middle of the circle facing the teacher, and he said, "Bus." I told the teacher, "Isaac would like to sing The Wheels on the Bus." I guess she couldn't resist such an eager listener, so we sang it. He sat watching her closely as she sang and made the motions that accompany the words, although he didn't open his own mouth or lift a finger.

3) It's not part of my standard repertoire, but on Friday I sang My Bonnie Lies Over the Ocean, and it turns out Isaac thinks it is the greatest song ever. He even asked for "more song," a request he has never made before (and as it sounded like "more hong," at first I wasn't sure what he was talking about). He has also demanded the song by name over the past few days -- he says, "Bonnie!" He always asks for books by their titles, but before this he had never named a specific song.

4) Today something inspired Craig and me to begin humming the main guitar riff from the Black Sabbath song Sweet Leaf. It's hard to reproduce it in writing, but we sang it a little bit like "Run-djun, run-djun-jun, jun jun." Isaac listened to this with great interest, and then said, "Run-jun, run jun." He said it tunefully, very nearly reproducing how we had sung it. Could my son's first attempt at singing really be the riff from a heavy metal song from 1971?