He is your nephew, you are his Aunt.
In English, there are distinct titles for relatives, such as aunt and uncle for either of your parent's sisters and brothers respectively. Cousin (whether male or female) is used for the children of your aunts and uncles such as they relate to you. Your cousins' children will be your second cousins. A great aunt and great uncle will be from your grandparents' era and adding step-family members to the mix can further complicate things. The names used by step-families, however, are unlikely to change, and if a distinction is necessary the word "step" is added as a prefix to the relevant word (for example, step-cousin).
In the English language, your nephews and nieces are the children of your brothers and sisters and this title extends to your husband or wife's brothers' and sisters' children. However, as the world has become like one large family, language differences have caused confusion and what is clearly understood in one language is confusing in another. The word nephew in English, as previously stated, means the children of a sibling or spouse's sibling. However in Afrikaans, one of the official languages of South Africa, there is a dilemma because the word for nephew is the same word as the word for cousin ("nefie" is the affectionate and most commonly used word for "neef," meaning nephew or cousin).
This creates communication difficulties in English when a person whose first language is Afrikaans inadvertently speaks in English about his or her nephew when he really means cousin. The English-speaking recipient of information relating to this particular cousin (or is it his nephew?) has no idea that the same word is used interchangeably and is often confused as to who the subject of the conversation is.
Here is a video further explaining family relationships:
In addition to a nephew being the offspring of a sibling, we also refer to male offspring of a spouse's siblings this way. My husband's sister's son is my nephew, too. For a niece, it is the same. The female offspring of my spouse's sibling is my niece.
A great-niece or great-nephew is one generation down, so your nephew's children are your great-nephews or great-nieces. You are referred to as a great-aunt.
The degrees of relationship in a family can be important for the purpose of inheritance and other for other reasons. We call these relationships degrees of consanguinity.
The male offspring of any one of your siblings will be called your nephew. The female offspring of any one of your siblings will be called your niece. You will be called their aunt, regardless of their gender.
Your sister's son will be your nephew. As his aunt or aunty, you should introduce him as your nephew, instead as your sister's son. If it was your sister's daughter, then you would refer to the child as your niece. Nephew is what you call your sibling's son and niece is what you call your sibling's daughter.