Hi, Newson2,
Oh, lucky you to be walking a camino with your son. I have never walked with my son, though at age 19 he walked the Camino by himself. Well, after the first day he wasn't really by himself and he found his camino family and had an amazing experience.
I agree with tyrrek that the best understanding to start off with is that each of you might "peel off" and find your own little camino bubble, but of course with the mother-son relationship being what it is, my bet is that the mother doesn't find her own own bubble unless the son has found his. If both of you have your own phones, you can keep in touch if it turns out that you want to walk with others some of the time. I think a lot will depend on what the "pilgrim traffic" is like when you are walking. If there are lots of young folk, your son will probably gravitate to them, if there aren't maybe you'll walk a lot of the way with him. In any event, I would think that the best idea is to go without any pre-conceived ideas, and a huge dose of flexibility, which is actually good advice for anyone starting the camino, but maybe even more important for people who are closely related who are starting the camino together.
Two years ago, I met a man who had started in Irun with his son, and by the time they got to Santiago, they were communicating daily by phone. They had a really emotional reunion in Santiago, I was there when the son walked into the square, so no matter how things go, and no matter how many kms you actually wind up walking together, it can be a wonderful experience for both of you.
Hope you will keep in touch and let us know how it goes. Buen camino, Laurie