What is the masculine of madam?

In English there are no masculine or feminine forms. English uses gender specific nouns for male or female.

Madam (abbreviated Mdm.) is a title used only in formal situations to address a female (Madam Ambassador, Madam Chairperson, etc.)

Sir is an appropriate counterpart to address a male in formal situations.

Mister (abbreviated Mr.) is the title for a male in less formal situations.

Mistress (abbreviated Mrs.) is the title for a married female.

Miss is the title for an unmarried female.

Ms. is the title for a female without marital information. The noun 'Ms.' is not an abbreviation, it is a Word created to eliminate marital status from the title of a female.