There may be some slight angulation to his legs creating difficulty when he walks, as the feet cross over each other when he steps. There are many condtions that may cause a bird to develop a problem; most predominantly that will be a nutritional-related deficiency. Your bird is quite old, and...
A video would be helpful in determining the cause of the leg problem. If you're noticing his are feet pointed inwards (Pigeon-toed), and he's tripping over his own feet, that may be suggestive of a niacin deficiency. Birds with such a deficiency often become inactive; sit around, develope wet...