I'm not a vet but I have 3 dogs and 3 cats so I'll weigh in based on personal experience. There are products that you can spray on wood that make it taste bad to dogs. It makes the wood taste very bitter. One product I can think of off the top of my head is called Bitter Apple. However, if it's a large area, that might not be possible. You could ask your local pet store what products they have to see if they have something that works for a larger area. I have had good luck with those products but the problem is you have to keep using them until your dog associates the bad taste with chewing the wood. Then, he may just turn to something else, so ultimately it might be worth finding a professional that works with behavioral issues with dogs to get some pointers and try to get to the bottom of what might be causing him to chew.
Important: The opinions expressed in WebMD User-generated content areas like communities, reviews, ratings, blogs, or WebMD Answers are solely those of the User, who may or may not have medical or scientific training. These opinions do not represent the opinions of WebMD. User-generated content areas are not reviewed by a WebMD physician or any member of the WebMD editorial staff for accuracy, balance, objectivity, or any other reason except for compliance with our Terms and Conditions. Some of these opinions may contain information about treatments or uses of drug products that have not been approved by the U.S. Food and Drug Administration. WebMD does not endorse any specific product, service, or treatment. Do not consider WebMD User-generated content as medical advice. Never delay or disregard seeking professional medical advice from your doctor or other qualified healthcare provider because of something you have read on WebMD. You should always speak with your doctor before you start, stop, or change any prescribed part of your care plan or treatment. WebMD understands that reading individual, real-life experiences can be a helpful resource, but it is never a substitute for professional medical advice, diagnosis, or treatment from a qualified health care provider. If you think you may have a medical emergency, call your doctor or dial 911 immediately.