Slower work that improves elevation can be better for muscle tone than a lot of fast work - so lots of serpentines and spirals and direction changes in his routine will encourage him to gather himself together to maintain balance
I also like to use ground poles and change the distances so they've got to shorten and lengthen their stride and lift themselves to get over them not just skim them
I'm wondering about his muscle loss and what looks to be a rather long coat for an arab that's stabled and blanketed - Have you ever had him tested for IR and/or Cushings?
Thanks for the suggestions! He is only blanketed in extreme cold (-20 C), though I am considering blanketing more often to encourage shedding. I brought up IR to the vet and my farrier and both say he shows no signs of it. All the horses around here grow really long coats. Many BOs clip for that reason.