It probably means that he hasn't checked his email, he doesn't want to go, or he's really busy. You'll have to check on him on that if you want to know. This could mean a variety of things so I can't give you a definite answer.
Also, you should try to avoid asking people out by email. Even though he's shy and you may not feel comfortable talking to him, it's best that you ask him in person. It'll show that you are mature and that you are serious about taking him out. These both increases your chances of him agreeing to go out with you.