Is Mr. Trump, President of "The United States of America", obligated to show his past tax returns?
In a word: No.
That No, being qualified, unless there exists evidence of criminal. filings. That is the job for the IRS.
Simply because previous presidents openly showed their returns, their actions do not make it a constitutional requirement for other presidents to do the same.
Leave the man alone.
I'm certain that if every member of Congress and all the US Supreme Court Justices wants to show ALL of their past tax returns from several years before their first day in office all the way through to the present, perhaps that would encourage Mr. Trump to follow suit.
It is not a constitutional requirement.
I really would like to see Congress repeal about 10,000 of the unconstitutional laws, bill, agencies, departments, etc they passed and created.