Some of us have to do that in order to learn. There is not a weapons kata I ever learned successfully with out making written notes and following them at home step by step over and over. There was not a single aikido "basic technique" that, when first learning the gross movements, I was able to learn the basics of without parsing them into tiny pieces and murmuring aloud what to do with my body.
I do find it curious that he has never stopped doing that, though. It may be that it has become pure habit and isn't actually necessary or it may be that he is neurologically wired to need to do that. Worth a VERY gentle exploration with him.
Very gentle indeed. We're good friends and I definitely want to avoid any suggestion of "You SHOULD stop talking because it annoys ME". The talking, though, isn't just (or even mostly) a recitation of the steps, it's more an analysis of what just happened (usually with accompanying berating of self). For me, as a partner, it feels like an interruption: he's interrupting the doing of the kata to talk (to himself) about what he just did. As a result, his partner practice is jerky and full of "zigged when he should have zagged" moments -- sometimes dangerously so, for him or for his partner. He knows that he needs to have a better focus when doing weapons, but as you say, the talking seems to be his method for trying to "get it right" on what my sensei calls the "Arthur Murray School of Dance" level: I move this foot there and this hand there, etc. The problem is that when a partner is involved, walking through the correct steps isn't enough.