What do you call an uncle's wife's sister's son?

Call him whatever his name is.

If your uncle is the brother of one of your parents, then his wife's sister and her son are not related to you.

If your uncle is the husband of one of your parents' sisters, then his wife's sister may be:

• your mother, in which case the son is you or your brother, or
• your aunt (sister of both your parent and your uncle's wife), in which case the son is your first cousin.