Hi, dbogen!
Come on over to the disABILITIES board for any questions you might have about getting around at WDW with someone on crutches. Southwest air will allow you to pre-board because your son is considered to be temporarily disabled. Bulkhead seats aren't always the best seats when a leg is in the cast, as there isn't that space under the seat in front of you. When you pre-board, remember that the crutches don't count towards your carry-on limit. I would suggest getting a seat as close to the front of the plane as possible and maybe even an aisle seat if he needs a bit more space.
As far as the wheelchair...mention it to the gate attendants before you board, mention it to the flight attendants when you get on the plane and mention it again to the flight attendants before landing. Plus, calling 48 and 24 hours before your flight to remind them that you will need a wheelchair, while not required seems to help get that written down somewhere(smile).
Hope you have a great trip!