I've made a number of quilts for boys and I try to use a sports theme if they like sports. I made my son an Alabama Crimson Tide quilt and done several others using baseball or football fabrics. I am making one for my youngest nephew right now with guitar fabric as the central theme because my brother is in a band.
What does your grandson like, what hobbies does he have, or what sports does he play? You can find something that will be just right for him.