What about mentally stimulating clicker games? Get a clicker, and pick some tricks with step by step instructions off YouTube. Start out searching for "Kikopup" and look at what she has taught her little Chihuahua. She has great instructional videos. They don't have to be complex tricks. 5 minutes, 2-3 times per day in between studying. This will tire him out as much or more than any toy or chew. It just might be that extra little bit you need in addition to all the great stuff you bought for him.
By the way... Don't use sit/stay in a place as a "punishment." A dog should love doing obedience exercises for you. He should sit/down with joy, and be rewarded for doing so. He really shouldn't need to be punished... If he is being a bit of a bother for you at the time, then perhaps a chew in his crate set near you while you study is better. And he should love his crate! If he doesn't, then that's a great place to start with tricks.