These older performance hoses commonly have arthritis and what you describe is really common. With arthritis, joints get stiff with inactivity but they loosen up with work. The most important thing you can do for him is to stop stalling him. Full turnout and light daily exercise are good for arthritic horses. No matter how many supplements, etc you use if you are stalling him up you are requiring that he stop moving around which leads to joint stiffness and pain.
Licensed Veterinary Technician