You make sure at each stage you've had him sign an approval form telling you to move on to the next stage. So, after agreeing a graphic design, get him to sign it off. Then go and code the template - get him to sign that off. Finally build the site and get it signed off. If he changes his mind after approving the layout, you tell him 'no problem, but as you did approve this layout I'm going to have to charge you £xx per hour to come up with a new one.
If he didn't sign any approval forms, make sure you at least have written confirmation such as emails to prove he told you the layout was fine.
Clients are free to change their mind, so as long as they are happy to pay for your time to change things, it shouldn't be a problem. Just make sure at each stage you communicate well with the client and ensure they are truly happy with the work so far. This way you can pick up on any potential issues sooner rather than later. Also make sure your contract makes it clear that changes to an approved stage will incur extra costs.