Welcome from Michigan!
So you want to make the same type of quilt for your son, Basically one piece of fabric on both sides?
Don't go with a single seam. Take 2 lengths of fabric and cut one in half sew the 2 pieces to each side of the uncut piece, centering it. If it's too wide, then trim a bit from both sides. When you make the backing, cut an inch or two from the center piece so the seams on the front and back don't line up.