Sigmund freud proposed that little boys’ fears that their fathers may retaliate against them for forbidden sexual and aggressive impulses may lead to

Castration anxiety

Explanation:

Castration anxiety is the central concept of personality. It is related to the concept of psychoanalysis. Castration is more linked with the concept of the Oedipus complex.

Sigmund Freud was the first person who has been proposed this concept. Castration is nothing but only the fear and anxiety about the castration of the genital organ.

It is a childhood experience that occurs between the age of 3 to 5 years. It is also called an unconscious experience that occurs again and again with all individuals in their childhood.  

Thus the little boy fears that their father would castrate his genital organ for the forbidden sexual aggressive impulse that leads to the castration anxiety.