I do note in other people's photos posted on All Trails of hikes on Amargos, that often the trail is next to the stone walls of the terraces. From these photos, many areas looks rocky but well trod. There are really no wilderness areas on this very small island, it's more like acres of ancient stone walls with pens or even windmills here and there. And goat trails, too.
The brother did complain bitterly that the US Consulate could offer no help in organizing a search and had no resources. I think mention was also made that shepherds were aware and were also alerted to search for him.
I think posters and a reward could be very useful.