As a tried-and-true Ravenclaw, it was apt that Kevin's lie would be caught out. Working from plans and guidelines was what Ravenclaws did. Fools rush in and all that. Kevin's retort had been only that: a retort.
But now he was caught slightly wrong-footed. He had expected, had planned for Drew to just do it, to kiss him or not. Playing the ball back into Kevin's court gave him a couple of options. He could say "yes" and see what Drew would do; he could say "no" and disappoint both him self and, presumably, the Hufflepuff; or he could improvise.
Although a fantasy in which he took the lead and made the first move danced across his imagination, Kevin's feet failed to cooperate with the idea, so he took the next-best course: