in what ways do you think his story would have been different if it had been told by a first person narrative from Santiago's point of view? Or from the boy's point of view?

I think the omniscient clarity of Hemingway would be taken away. First person narrative would demand the reader sift through the lament and pain that each character feels. The more objective third-person view illustrates the heroism of Santiago beneath his breaking body and his breaking little boat. Hemingway elevates Santiago to the status of Christ-figure: something that might be lost in first-person.