I pick a sports fabric for my main fabric for boys quilts and then build around it. Do they have a favorite team? You could maybe make a quilt using fabric from their favorite team.
Or do they have a hobby or interest that they enjoy, like astronomy? You could use a fabric with planets and stars.
I am making a quilt for my son using a number of golfing fabrics. He is an avid golfer and will love it. There are so many great novelty fabrics out there that you should be able to find something.