Try putting short strips of double-faced tape on the spot he likes to claw. The sticky tape annoys the cat and usually breaks the urge to scratch. Be careful to follow directions on the bottle to avoid possible danger to your cat. Learn how to trim your cat's claws, and do it at least once a month. It won't keep the cat from clawing the furniture, but it will limit the damage it can do while you are training it to use the scratching post.
Most pet stores sell nail clippers designed for cats.
A veterinarian or groomer can show you how to trim the nails without causing pain or bleeding. If you must have a declawed cat, consider adopting a cat that is already declawed. Almost every cat owner who really works at solving the problem will find a successful technique. We're just a phone call away.