The attack was made on an impulse--Gary had no plans beyond just punching Kenny really hard and hoping for the best--so he's in no position to dodge any of the kid's counters. The kick to his shin knocks him to a half-kneel, holding himself up with the overhanging mesh with one arm. This happens to be the arm that Kenny sinks his teeth into.

"Fuck!" Gary lets go of the mesh. Almost immediately he loses his footing on the thin planks and starts to fall through.