It would depend I think on his x-rays.... Much of the pain in NS is inflamation of the deep flexor tendon from moving over a changed, rough Navicular bone. The more the tendon moves over that rough surface, the more inflamed it gets. exercise improves blood flow and improves the condition, too much causes damage. It shows up in hard work, disappears with rest.
I'd just have to try doing what I'd like to do with him, and really read him as to whether he is holding up and enjoying the activity. Right now, he gets really jazzed about going for a ride and he steps lighter and faster than when he is clunking around the pasture all bored.
Do you know what is "too much" excerise?
Becky