I'm sure your grandson will appreciate a quilt made by you but I do have concern for the use this quilt will receive and with the time and love you will put into a quilt. I would not be so concerned about a quilt that will fit his bed. Dorm beds become something to sit on, study on, eat snacks, etc etc. They get pretty rough use so a purchased bed cover might be more "conservative". Making something as a throw that will "fit" anywhere might be better? If he will be in an apartment where this is not a concern, then your quilt might not get as rough treatment. I'd recommend neutral colors or something in his high school colors if they are something that will be liked in years to come.