Andrew didn't know whether to gasp or giggle at the feeling of Jack's thumb brushing his skin but his body made his mind up for him with an amalgamation of the two.
Jack was, oh, Jack was biting at his lip now and Jack could certainly have all of those sounds that he wanted because as long as he kept treating Andrew's mouth like that, Andrew didn't really have much of a choice but to keep making them. He reached to put his hand on Jack's neck gently, stroking his thumb there just behind Jack's ear.