A.L.
Verbal tic. Akin to saying "um" alot, or putting "like" in a sentence. I did it too, from time to time, even repeating a whole phrase. He's learning how language works, trying out what he's saying and how it sounds. He'll probably grow out of it.
BTW, is he a lefty? We lefties are more prone to stuttering, for unknown reasons. Just one of those things, I guess.