First and foremost, congratulations on your upcoming trip; it'll be here sooner than you know it! Secondly, while there aren't any specific programs aimed at children with Down syndrome (I have a twelve-year-old daughter with it), that doesn't mean that your son can't participate in most things. For example, since he's a lover of all things buccaneer, you might consider taking him to
The Pirates League where he can get his own pirate on. The
Pirates and Pals Fireworks Voyage is another great activity for children his age. My daughter loves every minute of it! Your son's love for Mickey and Minnie means he'd probably have a great time dining at
Chef Mickey's where the characters will come by his table for a short visit while he's eating.
I've always found that my daughter is more alike other children than different; for the most part, she enjoys the same types of activities that my typically-developing kids do. There are a ton of options for kids (and kids at heart) all around Walt Disney World. My suggestion would be to read up on some of these
tours and special experiences and see which--if any--your son would most enjoy. Have a great trip!