Nicky was, for lack of a better word, scared. Scared that he didn't want Justin to stop. And scared that at the same time, he did. He wanted to breath, to think; and at the same time, he didn't, he wanted Justin to smother him and overpower him with these rampant emotions. There were so many, fear was just the one that stood out the most right now. Fear that he wasn't doing the right thing, that he would mess up and turn Justin away. Fear that he was doing something right, and he would be Justin's next month conquest.
And still, he didn't pull himself away. He was incapable, tethered here so that wave wouldn't crash down over him. And, he liked it here. Finally, making his first move, Anthony put his hand ontop of Justin's, the one that was rubbing his knee, though he didn't know what else to do with it after that.
He was also still making little noises, whimpers, sighs, anything that would help to encourage Justin. (Which confused him ever more.)