Welcome aboard, Dawn! Just a few more weeks until you set sail on the high seas with Mickey and his friends! I sailed on the Magic out of Galveston in January and had a wonderful time, as I'm sure you will on your sailing as well.
On your eight night voyage, there will be one Formal night and one Semi-formal night. The other nights are Cruise Casual, (or pirate attire on
Pirate Night). Disney asks that guests wear long pants (not jeans), skirts or dresses in the main dining restaurants. Little boys, including your seven-year old, would be fine wearing dressy shorts and a polo shirt for Cruise Casual nights.
On Formal and Semi-formal nights, at least long pants like khakis and a dress shirt should be worn. Many families go all out with suits and tuxedos, formal gowns and dresses on those nights. Some guests won't dress up quite that much, and your son will not be turned away in the restaurants, but I would encourage you to see if he'll dress up more than usual just at dinner. You might convince him by letting him know that even Mickey and Minnie and their friends wear their best that night. Goofy has his tuxedo, Daisy is in her sparkly formal dress and the other characters are out for some really great formal photo opportunities.
I hope that information helps, Dawn! Please let me know if you have any other questions as you inch closer to your cruise on the Magic!