There was a question about this on the DISabilities board recently and got some excellent replies so it might be worth you having a look.
However, here is my tuppence worth:
My daughter is 12 and was diagnosed with ADHD at the age of 5. Her symptoms sound similar to your DS - impatient, bored easily, highly strung, quick tempered and can also be rude and speak without thinking (although she is not violent, thank the Lord).
Whenever we go anywhere that involves a long journey or any sort of planning, I always involve her in it 100%. A goo didea at Disney might be to give your DS a spare map, so he can follow on the map where you are heading. Tell him in advance what you will be doing for, say, the next hour and ask him what he would like to do. Plan in a treat for him (not necessarily food or drink but perhaps a ride or see the characters) each stage of the way and tell him what that will be.
Another thing to consider is diet. I don't know how ADHD is treated in the States but in UK children are recommended to follow a certain diet - this means cutting out things like E numbers (particularly colourings), Aspartame (found in diet drinks) and caramel-type stuff. Although my daughter now eats pretty well what she likes, she was on a very strict diet up until 6 months ago. Whenever she did have anything with E-numbers or Aspartame, you could see a difference in her behaviour immediately.
I hope you have a great time and your DS enjoys his time in WDW.