Yes, Donald Trump and all other millionaires have the moral obligation to remember that many millions around the world are enduring financial hardships and serious economic disadvantages and it's their duty as successful businessmen to contribute through generous donations to the charities of their choice. It's a golden opportunity to do the right thing in the eyes of God and their fellow human beings.
Donald Trump and other very rich people who have way more than they could ever need should definitely donate more to charity. There are a lot of people who work hard, but just don't have enough. If I were in Donald Trump's position, I would definitely make an effort to help those people.
No, Donald Trump should not necessarily donate more to charity. Charities are set up for those who are able and willing to donate needed funds, but it is up to each individual to determine how much money they can and should donate to charities as a whole or to which charity they feel best donating. No one has a right to tell any individual how much they should donate, or even ask what that amount is.
Donald Trump should not donate more to charity. What he does is a refection of himself and no one else. He should and does donate exactly what what he means to. No one should dictate what others are to do with their money. It is a personal choice no matter they person's wealth or status.