My DD broke her arm two days before her surprise trip to DLR for her 5th birthday. We were lucky that it didn't swell, so they were able to put her in a cast the next day. It was a full arm cast, all the way up to the armpit. I was super nervous, but we were able to get the waterproof batting, which was great. We still avoided getting it wet, but it was nice to have a little peace of mind.
As for rides, it didn't seem to be an issue at all. She rode pretty much everything she was tall enough for except for Goofy's Sky School. I thought that one just had too much side-to-side motion and my arms were sore when I rode it, so I nixed it for her. We went in January, so the water rides weren't really an issue. She rode RSR four times, Space Mountain 4 or 5 times....not a problem with the cast at all!
Oh, and if your son wants to do Jedi training, the cast shouldn't be a problem there, either. My DD fought Darth one-handed!
I did have a garbage bag in my purse just in case it started to rain or something, but I never used it. I was a bit worried on Pirates, as there is sometimes a splash. Maybe a center seat or garbage bag/cast cover on that one, at least for the first part. I'd also avoid the "wet zones" at WOC if you're planning to see that.
I feel like the characters were extra kind to my DD on our trip. Nothing too out of the ordinary, but the combination of birthday girl and full-arm cast definitely garners some sympathy.