There are several insect pests that can cause holes in leaves such as the cabbage worm and possible slugs. At this point, scout for these insects and hand pick. If the caterpillars are too large, the microbial insecticde Bt will not be effective. It is too late to use floating row cover this season.
Next time you plant be prepared to cover the plants with row cover, handpick, or spray with Bt. See our vegetable profile for more information http://extension.umd.edu/growit/insects/imported-cabbageworm