Hello....
Boxers can be famous for their insentient chewing....we had a boxer 5 years ago that we had put in our basement overnight because of guests (little boy was terrified of big dogs) He chewed right though the door...lol...it wasn't funny then.
There are a few things you can try as the expert above mentioned. Try mixing bitter apple and a bit of water and painting it on the area's he likes to chew. There are other things that can be tried as well....you can use pumpkin mixture diluted with water to lightly spread on it..(very lightly so it can not be seen) you an also do this with pepper or hot sauce.
If you are at home and see the chewing try getting a water bottle and squirt him in th face with it so he knows that is it bad...(while telling him in a firm voice NO..BAD DOG)
Boxers are VERY smart animals and learn very quickly when they have don something wrong. There is also the option of kenneling him...but he really wont be able to connect the two if he is placed in there every time you leave. Though you can try placing him in it every time he chews as a punishment he will link it together in that way.
I hope I have been helpfull....I am including a link that may help understand why they do this and advice from others that have gone though this as well, it is a good site of info all about Boxers....Tonya
Please: Click Here
Remember to find the blessing in all that you have.....If you have other questions please use the reply if you are satisfied please click the GREEN accept button...feedback is greatly appreciated... Good luck Tonya