Since it's for him, maybe your son would put up with inching around it for a few days. During that time, you could get your layout done. Then, you could remove the entire thing before sewing by either a) stacking each row, labeling it and moving it somewhere else for assembly and/or b) making a diagram as you pick up each row, and/or c)take a picture of it and then pick up the whole thing in a heap. Just an idea.
I do the first 2, normally, but next time, I'll use the ipad to take a picture. Probably will put some paper numbers to the side of each row, as I am terrible at sighting down one.
hugs,
Charlotte