Some exercises that we have been doing with him include 'push-ups' both in his sling and on his exercise ball (above). In the picture below, he is in a sling on a treadmill. He's not yet able to move his back legs much beyond twitching, but we are trying to get his muscles going and remind him what walking feels like.
He is moving his tail more, even wagging it a bit, and today when I took him in the hallway to do some sling walking exercises, I am pretty sure he gave a good push with his back legs. Way to go, Harrison!