Have you checked his teeth to make sure there isn't an issue there? If his teeth have gotten over grown or chipped he might not want to chew. I would not rub anything like apples on the wood chews, that would just encourage mold growth. What else do you have in the cage to chew on besides the house and lava rocks? Chins get bored if they only have the same things in the cage to chew on all the time.
If the teeth check out, I would try some different kinds of wood sticks to chew on, not the prepackaged ones from the pet store though, those most chins don't like. There are a few vendors on here that sell chew sticks if you check out the banner ads and classifieds section. Not only different kind of wood, but also different materials too. Other chew toys include coconut shell, vine, palm leaf, woven grass, cardboard (if he doesn't eat it), cholla, bamboo, pumice, sola, and loofah. I try to mix and match things on hanging toys to keep things interesting. You should also have a variety of types of toys, some toss toys, some horizontal hanging, some vertical hanging, some shredding toys, etc. Also it's best to change out the toys from time to time to reignite interest. Simply moving the toys in the cage can sometimes make them seem new again, or swap out what is in the cage for new toys. If the toys are still good when you remove them just set them aside to put back in next time you swap out toys.