I agree with everyone else....absolutely bring him in the house while you are gone!Either crate him, or give him a "safe area" in the house where he can't cause to much damage.Fill a Kong or two with treats...or leave him other "durable" toys...something he can't swallow or choke on, something so he can amuse himself.And if you can afford it...doggie daycare a few times a week, or hire a dogwalker to come by half-way through his day to come take him out....they will feed him, walk him...or just have play time.Whatever you choose for them to do.
But leaving an escape artist in the back yard is nothing but a recipe for disaster.Even if he wasn't an escapee, I still wouldn't leave him unattended in the back yard for 8 hrs! Boxers are very curious animals and the possibilities of what could happen to an unattended animal are too many!
I understand that you feel he would rather be outside....what dog wouldn't? But you chose him, and with the choice of having any pet...you have to make the choice of becoming a responsible owner also. And knowing that he escapes the yard while you're gone....he would NEVER have another opportunity!Believe me, I learned VERY well about an escape artist....I have a Siberian Husky....and there isn't a fence built that he can't go over or under. He will NEVER have another opportunity for that again...he's a blue-eyed devil! I had to go through an ordeal...and lost him for a couple of days...I realized he wasn't getting the same treatment from me as my boxer was. And when I got him back...he did dupe me a couple of times...but I realized the ONLY way he could be outside was with supervision...and that's what he got! Just remember, he'll have plenty of time to be outside when you come home....ya'll can have lots of play time together, and you can supervise him while he's outside. Otherwise, if you continue to allow him to be outside unattended, you may miss out on the chance to build a wonderful relationship with your boxer.
This is JMO...but I hope you make the right choice for your baby.Please keep us updated!