Hi,
After writing this post I discovered that this is actually a "Boost" theme issue and not specific to the "Fordson" theme.
It seems that the option to use submenus in the custom menus was removed intentionally from the "Boost" Theme:
1. https://docs.moodle.org/37/en/Theme_settings#Custom_menu_items
2. https://tracker.moodle.org/browse/MDL-56782
After writing this post I discovered that this is actually a "Boost" theme issue and not specific to the "Fordson" theme.
It seems that the option to use submenus in the custom menus was removed intentionally from the "Boost" Theme:
1. https://docs.moodle.org/37/en/Theme_settings#Custom_menu_items
2. https://tracker.moodle.org/browse/MDL-56782