You should tell his doctor. When a child eats wood, rocks, ashes etc, it's because their system is craving for or needing something.
You might have heard the dumb stories of pregnant women eating rocks. It's not a dumb story, they actually have eaten rocks of one kind. It was the ones that had a lot of iron in them. Their system was craving iron. That's from years and years back and since that time doctors found that pregnant women needed iron tablets during that pregnancy.
If you spray anything on the wood, he might stop chewing it asap but, it might harm or kill him asap!
Chewing on furniture is said to be caused by a mineral deficiency but should consult with the pediatrician and tell him/ her about it. You can stop it by making your son be more engaged in various activities such as playing with toys and have him watched constantly to keep him away from chewing on wood. Note that the wood chewing can be harmful to him.