Some dogs tolerate the chondroitin fine, but to be safe, I would first try straight
glucosamine, maybe about 500 mg a day. Give it every day
for a few weeks, and see if that helps. In the meantime, you can try an enteric coated low-dose aspirin tablet (80 mg like they sell for heart patients, but be sure you get the enteric coated), once a day with food. If the glucosamine works, then you can discontinue the aspirin after possibly as short a time as a few days.