Manolin (the boy) follows Santiago (the old man) because he sees that he is a man of honor and courage. Manolin knows that other fishermen have newer equipment, larger boats, and more men, but not the pure skill and learned experience that Santiago possesses. There is a difference between fishing for food and sale, and fishing because it entails the entire heart and soul of a man's life; Santiago's life is in his work:
"Que Va," the boy said. "There are many good fishermen and some great ones. But there is only you."
"Thank you. You make me happy. I hope no fish will come along so great that he will prove us wrong."
"There is no such fish if you are still strong as you say."
"I may not be as strong as I think," the old man said. "But I know many tricks and I have resolution."
(Hemingway, The Old Man and the Sea, Google Books)
By the end, Manolin sees how Santiago has refused to give in, refused to let the sea and the ignorance of others influence his actions. Manolin learns the value of persistence, of pride (in its proper place), and of willpower over weakness. Manolin resolves to learn everything that Santiago knows so he can become as great a man as Santiago, and so he can pass those lessons on to his own children in the future.
The Old man gives us the lesson that :
A man can be destroyed but not defeated.
same lesson Manoline gets from Santiago.