Sa tête ou la tête?

"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.

Hi Lori :)
I'll try to answer this, please correct me if I'm doing wrong.

From my understanding, the right answer is "sa tête" because we are talking of "his" head (his ownership), also, we are talking for 3rd person pov hence we use "sa". You can look back to the link bellow as a reference:
Mon, ma, mes; ton, ta, tes; son, sa, ses = my; your; his / her in French (French possessive adjectives)

While, "la tête" is more when we are talking aboud head in general without any reference to whom is "this head" belongs to or describing a body part.

I hope my explanation makes sense.

Wish you a lovely day :)