If you are a guy, your relation to your father's brother's son is that of cousins. You and your father's brother's son are both the sons of your respective fathers, which makes you first cousins.
In family relationships, first cousins share the same grandparents. In this case, your father and your father's brother (your uncle) have the same parents, and you and your father's brother's son have different fathers but share the same grandparents on your father's side.
Cousin relationships are commonly found in family trees, and first cousins often have a close bond growing up together in many cultures.