Well, this is one thread I can definitely help with
DS6 broke his arm (right at the elbow) exactly 2 weeks before our first ever trip to WDW. We were there 5/20-5/26.
I was crushed and so worried about how he'd do and what limitations he may have. Our situation was a little worse b/c the dr would NOT put a waterproof cast on him b/c of the nature of the break (near the elbow). Again, I was very upset. His cast went from above the elbow to his fingers.
Well, guess who was the ONLY person worried about this - yup, ME. He had a BLAST. He rode EVERY single ride, including Space Mountain and Expedition Everest. NO ONE said anything about the cast.
In our case, the dr was/is very concerned about range of motion, so the day before we left, he was taken OUT of the cast even though the fracture was still healing. They cut the cast longways, to make a splint - so the cast was still there on the backside/ underneath his arm. Then we wrapped an ace bandage around it. Then he was to take it off 3 times a day and practice moving it. The day befoer we left, his first time out of it, he cried so hard b/c it hurt. I called the dr crying saying this would ruin our trip.
If I were you, I'd do a few things - first call the dr and make sure s/he knows you are going on vacation and your son will want to swim. If the break has healed enough by then, he should be able to get a waterproof cast. Depending on the severity and location of the break, he might be able to get one.
He NEVER once complained about his arm once we got there - things didn't work out well in the pool b/c we ended up having to take it all the way off and that made it sore, but other than that, the trip was AMAZING!
I totally understand how you feel - I was so upset about this.
Oh, and just wanted to add that we went to Sea World and their workers told me he couldn't ride the roller coaster in the kiddie area b/c he had a cast above the elbow. I told them he had ridden all the bigs ones at MK, but they still said no - when I explained that it was a "splint" not a "cast" (even though it was a cast mostly), they let him ride. So they do have rules there.
DS is now out of his cast full time, but we have to go to PT two times a week and he has to do exercises daily. Hope your break was not that bad!