There is nothing listed about accessibility for the new ride at Typhoon Lagoon in the
Guidebook for Guests with Disabilities for that park. I would expect it to have some access (like an elevator) for people with disabilities (like was already mentioned), but I don't know for sure.
I do know for sure that all of the other slides at Typhoon Lagoon have steps (and lots of them) and none of them have elevators for access.
I know that Blizzard Beach does have some slides with elevator access (there is a link to the Blizzaed Beach Guidebook at the top of the one for TL). Other members of your party need to be responsible for getting the wheelchair/crutches back down.
Another thing to think about is that there is a lot of sand at a water park. If you stay on the paths, the tips of the crutches won't sink into the sand. Traction/slipping can be a problem though because of sand dragged onto the paths and be careful of the path edges. (I don't know from your post if your son is a temporary crutch user, who might need this info or a regular crutch useer who would already know this).
They do have wheelchairs to loan out at the water parks; inquire at Guest Services. Using a wheelchair would be probably be easier than crutches.
Also, depending on his reason for using crutches, you should probably talk to his doctor about whether going down a water slide is a good idea or not.