Gryphon, my big German Shepherd, absolutely destroys Nylabones. He hacks off chunks and bolts them.
I get him a raw knuckle per week and he gets it 3 times a week for an hour at a time when I can watch him.
Keeps him busy and at seven he's never needed his teeth cleaned. Another thing I do is to get whole chicken backs and freeze them. One of those makes a nice doggie 'popsicle' and the bit of extra animal fat is good for the coat.
Again, you much supervise.
Even if you get the "super" Kongs, you still have to watch them. I get a few months out of a Kong before I have to replace it.
It's a PITA, but until his back went bad (spinal stenosis) Gryphon was a very active working dog (tracking and obedience), and he just wasn't coping with the enforced lack of activity.
He didn't become destructive in the house, but he started licking and chewing his paws and the like. Poor guy was just bored to tears.\
A very wise dog-trainer in Germany once said to me, "What do you expect them to do? They can't drink or smoke, and they can't take up knitting!"
If you want to go the treadmill route, talk to your vet about it. Your dog should be checked out before starting an exercise program just as you should. My niece has a pitbull kid that does wonderfully on a treadmill.
Gryphon is too weak in back to use a treadmill safely. Remember that if your dog is of a working/herding breed, that they are designed NOT to let you know when they can't handle exercise.
Boxers are actually darned good tracking dogs given the pushed in face and some of them do very well in agility. Ace might really enjoy that sort of thing. Another thing to look into is whether he is calm enough to do therapy dog work.
Gryphon's GSD predecessor was both trained in protection (Schutzhund) AND had a therapy dog certification.