Complex sentence: As eldest son, James Matthews will inherit the courtesy title of Laird of Glen Affric upon his father's death, until which time he is able to use-should he choose to do so-the title James Matthews of Glen Affric the younger.
Split into simpler sentences: As eldest son, upon his father's death, James Matthews will inherit the courtesy title of Laird of Glen Affric. Until that time he is able to use, should he choose to do so, the title, James Matthews of Glen Affric the younger.