"he'd rehearsed in his head" 

I think in this query you are alluding to the way body parts are identified.

"sa tète" is appropriate here because it is a specific situation (his head) not a general comment.

https://progress.lawlessfrench.com/questions/view/using-le-la-les-with-body-parts-and-clothing-definite-articles

The above link may help you further.

Bonne continuation

Jim

In this case, I believe both are possible -- sa tête and la tête. However, using the sa tête in this case is, in fact, more common. Why? I honestly don't know. None of the rules I could find in the lessons pertains to this situation.