Okay, I've taken the suggestions to try training some new exercises to heart and we have been working on something new this week. (Thanks Hsin-Yi :) I think maybe he is getting bored with the repetitive obedience training, and it is time to add some new challenges to the mix. I can see already he's having more fun in training learning something new - especially with clicker training where he gets lots of treats and he's not worried about making a mistake.
So, the exercise we've been working on is practicing rear-end movement with the box. I've seen several videos with different types of dogs doing this exercise, but had not tried it yet with Jackson. At the first session it took him awhile to put his front feet on the box. However, once he did that it didn't take him long to get the hang of it. The video here is the fourth session where we've started moving the back feet around while his front feet stay on the box.
I'm thinking when he gets good at moving his rear end with me standing in front of him it will be time for me to move to heel position and have him pivot both directions. Then, I'll have to think of other things I can do with the box - or with something else new!