What is the masculine form of the word Frances?

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

The name 'Frances' is usually the name of a female.

The corresponding name for a male is 'Francis'.

The most common spelling for a surname is 'Francis'.

Note: This is only a convention since the names originate from languages that do have masculine and feminine nouns (for example Francesca and Francisco). It's the option of the parents naming a child to be conventional or not.