Love the quilt!
Use a 'Boy' fabric for the backing. Then he will be happy with the quilt while he is a child and when he grows up. That's what my Mom did with my grandsons quilt.
I made a quilt that my 6 year old grand-nephew claimed because it is his favorite colors - blue and yellow. But the fabrics are very adult. So I followed my Mom's example and pieced together a 'boy' backing for him. He loves it.