Just my opionion, but finish it the way he intended it to be made. If some of the seams are too close to the edge, back them with fusible to stabilize them, but don't unsew them. I wouldn't add anything (you mentioned flowers in the corners) to the quilt except for what your son already had cut. Granted, if you don't have enough fabric to complete the quilt, try to match his fabrics as closely as you can. I always loved browns and teals! This is going to be beautiful!