How does the old react to the physical pain in back of his hand? What does this tell us about the character of this man?

There is actually a lot to this. The man reacts to his ailing body as sacrifice. Hemingway turns Santiago into a Christ-figure. Santiago's hands are cut by the fishing line. This combined with old cuts are symbolic of the suffering of Christ and his stigmata. When the sharks ravish the meat of the Marlin the meaning of Santiago as Christ-figure deepens. He moans in agony as if he is "having nails driven through his hands." Santiago's defeat is man kind's defeat yet in his defeat there is a sense of redemption. He returns empty handed (except for the skeleton) yet he has attained what others once laughed at. The boy Manolin tends to his wounds like he has been taken down from a cross.