Goodness, I don't know I can offer much advice for your grandfather with his leg in the air!!!
Just kidding.
Try sitting on a cushion, just under the sitting bones, not your whole bottom. This will elevate your tailbone and bring your knees closer to the floor.
Janu Sirshasan (head to knee) will help you, practice more on the stiff leg.
there's something else I'd like to recommend. I'm searching for the link. More later.