Well, assuming you've tried all the "short rides to the park" first, then pretty much anything yummy he can chew on will work to keep him quiet.
But I get these things at the local "holistic" pet store - "Pet N Shape Fine Chicken Treats" - I found them online here -- OK they're called "chicken skewers" but I like chicken lollipop better
http://thepuppyshop.com/chickentreat2.html
They're expensive but you get a bunch of them in a package and he goes through one every few days - make sure he is sitting on a towel or something because they're a little messy - they also have these "Duck & Rice Dumbbells"
http://thepuppyshop.com/duckdumbbells.html
I also find if I give one to him when he's in his bag he totally ignores everyone and no one knows he's in there!
Let me know if it helps!