In the sentence "Hector called __ son, Scamandrius," the correct choice is "his." So, the complete sentence would be "Hector called his son, Scamandrius."
While "him" could also seem reasonable at a glance, it wouldn't fit correctly in this context. "His" is the possessive form used to indicate that Scamandrius is Hector's son, while "him" is an object pronoun and wouldn't work here.