The son of a first cousin is known as a person's first cousin, once removed, with the 12th degree of relation. Likewise, the son of a first cousin, once removed, would be the person's first cousin, twice removed, and the son of a first cousin, twice removed, would be a first cousin, thrice removed.
When a person's sisters or brothers have children, those children are considered cousins. The children of first cousins are second cousins to one another, while their children are third cousins. The child of a first cousin and the child of a second cousin would be third cousins. These titles can become even more complicated with stepfamilies and remarriages.