The narration is impressed with Sinclair for getting over the grossness of the shoulder in question enough to make such a fatherly gesture. The shoulder he squeezes is tense like a spring, shaking like a body with too much feeling and no clear direction for it.

Jack lowers his head slowly.

That wasn't a yes or a no. But the look and the tone were about right.

"Okay, mister Sinclair."

He doesn't sound miserable or happy, one way or the other. He just sounds agreeable: the one interpersonal skill he can always fall back on.