Peanut butter Kongs, bully sticks...you have to find out what will engage him. What exactly is he doing? Dogs that like to play fetch will play until YOU drop. I watch a dog that doesn't bark - but SCREAMS so loud when he's excited he's set off my burglar alarm (apparently he's the same frequency as glass breaking). I have thundershirts - I don't really think they work, but sometimes I'll try anything.
I watch mostly smaller dogs because of my cats, and you're on the opposite end of the spectrum, but I'm very familiar with the bigger ones, and they seem to have more anxiety. Pitties and GSD's are the biggest babies ever (I'm just guessing) - maybe he just needs cuddle time (I know, you're a guy, you can't admit that), but try!
Don't be afraid to ask the owner for ideas...I usually do it in a non threatening way, so they don't get worked up and think their dog is all stressed out.
I watched a shep mix for a friend before, and went to the dollar store and bought a bunch of stuffed animals because he liked to destroy them - it gave him something else to do (he didn't eat them, just ripped them to shreds), and everyone was happy. Get creative!
Yeah, I just figured out the formatting for this...sorry for the question in the title!