DeKay's are specialized slug eaters and will feed almost exclusively on them when they are young and small. Jeff mentions that they can feed on worms when they are older and this is certainly true (smaller worms, blood worms, may be taken when they are young). DeKay's snakes have specialized jaws and teeth that assist them in pulling snails from their shells. You could also try waxworms, I have no experience with this prey item but it seems to me that the snake might accept these.
The key is finding suitably sized items for such a tiny snake.
I have read that pill bugs (isopods - tiny land crustaceans) and small amphibians will be taken (were talking, perhaps, newly metamorphosed Spring Peepers - very small, an impossibility to locate in the wild).