How does Delayed Entry Program work?

I want to join the Navy as a corpsman, the recruiter said i'd have to join DEP and pick 5 jobs. He said they might give me one of the other jobs that i dont want, but he'll reclify me to corpman before i leave for bootcamp. I dont know if I trust him. If he doesnt reclify me, can i not join until he does? When am i "bound" in contract with the navy? is it when i join the dep? thanks for your help