You mention that he had other tops that his grandmother had done that no one else knows how to quilt ...
Why don't you just tell him to pick out one of those that he really likes, or has a special connection to, and you would gladly quilt that one for him for free .... and label it with his grandmother's name as the piecer and him as the recipient ....
If he doesn't want one, perhaps his dad would, since his dad may have more memories of the quilt's origin.