If you wanted your son to wear a tux, that would be best for formal night (Sunday). You will see everything on formal night, whole families dressed in gowns and tuxes, right to families wearing shorts and flip-flops. Do whatever makes the cruise fun for you. My daughter and I both wore floor length gowns while my son and husband wore dress pants, a button up shirt and a sports coat. Semi formal night is more like khakis and a nice polo shirt or button up, and for females, sundresses. But again, you will see all ends of the spectrum. The same is true on pirate night. You will see some people completely decked out as pirates, some people with just a pirate T-shirt and some people who wear their every day clothes. It did not seem like anyone felt out of place due to what they were wearing on any night. Just make it fun for you and your family!