My 4yr old son still chews on all kinds of things. He uses the chewing to sooth himself and to express emotion. We got him to stop the drywall corners by liberally applying pickle juice. Vinager had some effect as well as banana. His crib and windowsills look like a starving beaver lives there. He has chewie tubes and chewie T and P, he could care less about them.
He doesn't actually 'eat' stuff now, just wants to chew. My father-in-law has an oxegen tank and our son loves to chew on that tubing. He also loves clothing and blankets.
I redirect when I can, but its challenging.

My Husband spent 6months making me a custom mahogany rockingchair with beautiful oak and walnut inlay. It now has bitemarks all over it. You come to a point where you just have to accept that your life is different now. That material things are always going to be coming and going. Sensory issues are probably a good thing to look into. My most fav is "10 things every child with Autism wishes you knew". this is great for talking to family about.

I patched up the walls and repainted some of them. I guess that'll just be one of the new things, repairing as we go. I've removed most of the chair rail in his room and will replace with a flat painted white stripe eventually. Right now there's an uneven line where the sand and ocean met behind the rail. My wife worked so hard on that room.

if its for emotional reasons there could be something making him nervous. It could be the simplest thing like sounds or changes or even the presense of someone suddendly inthe same room. Its like talking to you without words.

Try a chewable toy/fidget such as www.kidcompanions.com our teething bling by smart moms availible at www.amazon.com