All good advice so far. I agree with trecile about stopping at Orrison if you start at SJPP. I would stop there, too, if it were my first day's walking and I'm not yet in my 70s. I also agree with SYates about considering starting in Roncesvalles. I started there on my two Caminos Frances (the first because that was where I was told it started and the second because I was retracing my steps with my son and so started in the same place) and it was a fine place to start.
Tincatinker's advice about having him join you for the last week is also good. Ultimately, you don't know where the "low point" will be. Physical, emotional, mental, or spiritual, it can appear at any point in the Camino and is tough if not impossible to predict. But even if your son is not there to support you through it, chances are very good that there will be plenty of other pilgrims from whom you can draw support, as well as your own internal resources. Having your son with you to share your moment of achievement as you finish your Camino is a more predictable goal and probably more enjoyable.