I think I did this as a child or at least a version of it. If I had to do something i.e. walk home from school, I would narrate the steps needed to complete the action as I did them. I'd often view myself in the third person and I think I said things to myself like "Eyre is putting on her shoes" just like your DS is doing.
Looking back on it I think I did this as a self calming strategy for when I was anxious - I didn't like walking home by myself and if I narrated it like a game or movie it helped me calm down and helped me feel like I was getting closer to actually finishing the task. Sometimes when I got home I'd continue doing it just because I was stuck in that mode. I did feel a little odd about it. I'd also narrate mundane tasks that I had to do. When I was totally engaged and enjoying something the narration would disappear because I was to involved to step outside of the experience and narrate it.
Not suggesting that your son is experiencing anxiety or anything. Just wanted to say I did this as a child and I did grow out of it.
