I broke my foot first day of my last WDW trip! The Celebrations ER was OK, but crowded!
Since I didn't have an able-bodied person to push me around, I ended up renting an
ECV. If your son's light enough and you've got an able-bodied adult who's free to push all day, a manual wheelchair might work better for you. There are a lot more hills and ramps in the parks than you realize when you're on your feet, so it takes some pretty serious upper body strength to wheel one's self - I tried it for half a day and had to give up.
Personally, I don't recommend trying to go with just crutches. The average WDW guest walks eight miles a day, and they really add up when you're not feeling well. I kept my crutches with me for places the ECV was hard to fit, and just parked the chair outside.
If he's got the Velcro-boot-brace arrangement, you shouldn't have to worry about getting it wet on Splash Mountain or other wet rides. I'm not sure about fiberglass casts - you'd have to ask the doctor.
If he's taking prescription paid medication, I recommend taking it before he thinks he needs it. (it's a small world is pretty trippy on Vicodin.) I didn't end up needing ice (for ice packs) in the parks, but I hear they'll give it to you at counter-service restaurants if you ask.
Good luck - it takes more than a broken bone to ruin a Disney trip!