Carol fig trees don't need to be thinned and chances are it's not age if your son planted it because most fig trees live for well over 50 years some over 100. There could be a few things going on. first off I notice you are in zone 10b in cali so the fig wasp is present in your area meaning many folks in your area grow the Smyrna fig wich is a type of fig that requires pollination in order to produce fruit. Without proper pollination, the fruits of Smyrna fig trees will drop from the trees before they are fully developed. Some commercial growers place baskets of caprifigs, along with the fig wasps that the small caprifigs are hosting, near their Smyrna fig trees to ensure that pollination will take place but they will travel to find and pollinate figs. If someone was growing a caprifig near you and it was removed you could be lacking pollination. Another thing could be you have a San Pedro type that also depend on pollination from other trees, but not to the degree that Smyrna figs do. San Pedro figs produce two crops of fruit each season. The first crop (known as the breba crop) grows on old branches, and it develops properly without cross-pollination. Later in the season, the trees produce a second crop of fruit on new growth, but this crop will usually drop from the tree before it matures if pollination hasn't occurred. If your son prunes yearly he could be removing his wood needed to produce the breba figs. Another issue could be too much nitrogen fertilizer producing more growth and foliage than figs. Hope this helps.
