Try this:
Begin in-hand, at the walk on a circle to the left, b/c on a circle you exert control on the hind legs when you ask the horse to yield his body. Face his left shoulder directly--90˚ off--with the lead in your left hand--your leading hand--and a dressage whip in the right, your driving hand. You will be asking the horse to move forward AND sideways. Slide you right--driving--hand down from the shoulder to where the leg would be if you were riding. Press lightly--as lightly as you want to ultimately use in the future--to ask him to move away from you. The moment he moves away--or even tries--relax and reward him. If he doesn't move away from the light pressure, use a bit stronger pressure. Poke him with a finger if you have to, but start with a LIGHT pressure before you get stronger.
As he begins to yield from you, move with him. If you stay stationary, he will have to turn around on his forehand. You don't want that. Stay on the bending line of the circle, just let it get bigger as he moves away from you. Do this on both the left rein and the right rein.
When he accepts your request to yield from you on the circle, try yielding across the arena. Begin at the walk on the left rein, on the long side. At the corner, begin making a circle to the left. After a quarter circle, maintain the bend with your leading--left--hand and use the driving--right--hand with pressure where the leg would be right behind the imaginary girth. Turn your left shoulder to the horse, which puts pressure on him to move sideways, and ask him to yield along the diagonal to the center of the opposing long side of the arena. Look in the direction of movement.
As you approach the long side again, straighten him for several strides and then halt.
Once he becomes used to yielding in-hand, you can follow the same steps under saddle. Remember, be patient and reward even the slightest try on his part.
Let us know how it goes.
ETA--I just re-read your post, and you said that he is stiff, and an older horse. Before you try what I suggested to get him to yield from your leg, have a vet look at him to see if his refusal to yield his hip is due to pain. It seems rather suspect that he won't yield his hip without bending his head to your knee, and that in itself is not good for a stiff horse. Call your vet.
