The relation you have to your cousin's son is that you are his aunt or uncle. This is because you are of the same generation and you and your cousin share a blood parent from the same family. The same relationship still applies if you are not fully blood related.
1 Additional Answer
The relation to one cousin’s son is the 1st cousin in-law. Your first cousins are the people in your family who have two of the same grandparents as you. In other words, they are the children of your aunts and uncles.