Our kids were picky eaters until the year we took them to the Food and Wine Festival at EPCOT. As we wandered around World Showcase we bought something from each stand. The rule was that they each had to take one bite but no more than that. If they didn't like it they never had to eat it again. Well...there was only one thing they didn't like (a veggie-filled crust that was very soggy) and boy were they surprised. Since then they have been willing to try anything and now are very adventurous eaters.
The cruise is a great place for your DS to start trying new things as the food is included in the price. I would suggest ordering a few basic things from the appetizer or entree menu and let him try them...with the rule that if he doesn't like it he doesn't have to eat it. Talk to him well before you leave and tell him of the plan and ask for his input on what he'd like to try. Maybe you can set an initial goal of three new foods...or one new food a day. Include foods from the ports that you might not be able to get at home. Oh, and make sure he knows that his favorites are there for him as well. I have found that if you make it a fun, learning experience kids are more receptive.
Have a great time on your cruise!
Michele