Perhaps starting in Barcelos would be more suitable. You will have the Pilgrims' Blessing service, which is a great way to establish the historical - if not the religious - significance of the journey, the wonderful bacelhau, then the route out of the town on a main road populated by giant Barcelos cockerels - see photo. Otherwise, starting from Ponte de Lima, but that involves a steady climb which, probably, is best undertaken after a preparatory couple of days of walking. Buen Camino! This is a wonderful adventure for you and your son.