Masses on Dog’s joints

In April, a mass appeared on my dog’s front leg, near his wrist. He was whining and not walking on it. It was hard and affixed. We got it x-rayed, and it’s not broken. Since then, he’s had two more appear, both on his back legs near the joint. He is limping and not putting weight on his one legs.

The vet originally thought a sprain, then broken, then a tick borne illness. We had a muscle biopsy done screening for cancer, and it came back clean.

Rimadyl helps his pain and he will walk on it. He’s been on multiple courses of antibiotics to see if it was an infection.

He’s a 5 year old German Shepherd mix (husky/dalmation)

Any ideas on what it could be? He’s been to 3 different vets and 2 others have been consulted with no real progress besides pain management.