Pick a background color you think he would like and that looks nice with the colors of the shield. Cut the background fabric whatever size you want it to be. It could be square or a rectangle depending on the size of the quilt you want it to be. Buy fabric in the colors of the shield. You will use the picture to cut the pieces for the shield.
I'm pretty decent at applique and am confident you can do this. Feel free to PM me if you think I can help. I did a "custom" quilt for my son last year, too. I know how important these things are to a mom.