Your cousin's husband is commonly referred to as your "cousin-in-law." However, the specific term can vary based on cultural and regional differences. In some places, people may simply refer to them as "cousin," even though it's technically not the correct term.
If you want to use a more specific term, you can call your cousin's husband by his first name if you are on familiar terms. For a formal or respectful address, you can use "Mr. [Last Name]." Ultimately, the most appropriate term may depend on your family's customs and the relationship you have with your cousin's husband.