Andrew didn't pull back when James kissed him, but he did go still for a moment at first. Yes, he knew it was coming, but that didn't mean he knew how to react initially. It was just a few seconds of stillness before Andrew let his eyes slide closed and leaned towards James just a bit.
And then the next thing Andrew knew James was nipping at his lip and that actually felt really, really good. It was sort of all he could do to not make any sort of sounds while James was kissing him, because he wanted to. But he was determined not to. James may have already known that Andrew was pretty much inexperienced in all these ways, but he didn't need to go and act the part. Not that he was suddenly going to pretend to be some sort of big slut or anything. That wasn't in Andrew's plan at all.
But what Andrew did do was hesitantly bring up his hand and let it rest on the back of James' neck.