Good for you for taking that kitty! I know that cats that are timid can take a bit longer to adjust to a new home but just give him some space. I would make sure he's got food and water (which you are doing) and just talk to him when you are in the same room. Trying to force him to come out will only freak him out. Once he sees he is safe there with you, he will come out and make friends, I am sure. And you know, little treats may help as well... good luck and thanks for rescuing Simon!