Hi, Jane! Lots of good ideas here. I like Patrice's idea of tacking by taking loooong stitches in a matching thread.
Crazy quilts used to be done by tying from the back, catching only the seam of the front. It appeared there was no quilting. I thought about that until I realized your backing has no seams... (So I did not read your explanation too well at the beginning. Sorry.)
Have fun with the process. It will be a great gift to your son.