First of all, please don't smack him.

Second of all, I feel your pain. I, too, have a chewer - more specifically, a wall chewer. Our dining room (which became "The dog room" when the dogs arrived) looked like the scene of a particularly violent shoot-out at one point - little horror had chewed so many holes in the plaster...

Make sure the dog has plenty of things he CAN chew, and rotate them often to prevent boredom.

Fill kongs/hollow bones with food/treats. You can use the dog's normal food so he has to work for it rather than it coming in a bowl all the time. If it's wet food, fill the bone/kong and freeze it to make it last longer. Ditto dry food - wet it, mush it up and fill the bone and freeze. The freezing also soothes sore gums (they go through a second teething phase at around the age your dog is, I believe - or it could be a bit later. Anyhoo, freezing makes it last longer). A smear of peanut butter inside a bone or Kong will keep them licking blissfully for ages. Or cream cheese. Whatever you can stuff in that they like, really

I found anti-chew sprays pretty useless but what DID work was Vick Vaporub - it's thick enough so that it doesn't evaporate or wear off too quickly and they HATE the smell/taste.

Does the dog get enough exercise - physical AND mental?