Billy's cock was already beating to life, rising in anticipation of Joe's tight, sure grip. Pavlov's fucking dog, Billy thought, I'm the dog and the zipper is the bell, and I don't salivate, I harden...

From Joe it was possessive. Lustful. Longing.
Aphrodisiac.

I loved those two bits the best, but the whole thing is smoking hot! I love Billy's thinking that Joe's calling him "Billy, my boy," is aphrodisiac. There's just something so compelling about a Billy who would think so.