There is no law requiring the president nor any private citizen provide their tax returns to anyone. the IRS is a specifically band from providing that information to the public.
They are however required by law if requested by a governmental body such as the senate for instance must produce tax returns of any and all years requested of any private citizen including the president. and that is exactly what Democrats are now requesting of the IRS formally and the IRS will have to comply.
however, the information may not be divulged publicly in any way, we will never find out unless it's leaked, the information requested of the president's tax returns via the current request to the IRS.