It's pretty typical for them to lose interest in the sticks once they strip the bark off. I usually toss and replace my chin's sticks once she's stripped them.
Have you tried hanging toys? Sometimes it's fun for them to chew on things attached to the ceiling or walls/doors. Just make sure they're strung with metal wire and have metal clips, as string and plastic aren't safe. You can even buy your own stainless steel jewelry wire and make your own hanging toys!
I usually like to make sure my girl has a nice variety of both hanging and loose chew toys. She mostly seems to prefer the loose toys, but she does still chew on the hanging ones. And every chin is different, some seem to prefer the hanging ones.
Some other safe materials my chin likes to chew are bamboo, coconut shells (with all the outer husk/fiber removed) seagrass braids, pear sticks, blueberry sticks, and chola wood. Just make sure anything you give them is organic, pesticide-free, and properly dried. Anything you buy should meet those requirements, that's mostly just something to be aware of if you make your own.
Some good places to buy this stuff are
Where Every Pet is Unique- Safe Handmade Pet Supplies by PandamoniumPets on Etsy and Farmer Dave's pet supply:
Homepage
Is your boy eating both his pellets and hay? If he's not eating well, then his lack of interest in chewing could be due to an issue with his teeth. If he's eating fine, then maybe he just hasn't found anything that strikes his fancy yet.