He is probably saying sounds that he likes, especially if the sounds are like words, but are nonsensical. It may be useful in his development of coherent speech, like the babbling stage. Also, it could be a form of verbal stimming. I'm 62, a retired research scientist, and I still sing songs with meaningless made-up words, that I replace the original lyrics with. It's very enjoyable for me.
Thinking about my nonsense singing, it may have something to do with liking Kenneth Williams, as Rambling Sid Rumpo, singing songs like 'The Ballad of the Woggler's Moulie' and 'Green Grow my Nadgers O'.
