Just to give a slightly different point of view... I spent a week in WDW last year on crutches and it was totally doable. I had injured a ligament in my ankle so was put in a walking boot but not allowed bear much weight, so basically using the crutches but able to lightly tip the front of the boot off the ground as I "walked". If your nephew can do that then it is so much different than full non-weightbearing hopping on crutches. I had to go non-weightbearing hopping with a different injury on the same foot a few weeks later and that was much harder - I don't think I could have managed WDW like that.
But just in case he is similar to how I was here are my thoughts on WDW with crutches (while in a boot that you can tip off the ground LOL - quite specific situation but you may find it helpful)...
I really didn't want a wheelchair or
ECV, I love strolling around WDW so wanted to be up and about walking at my own pace and because I was staying solo I didn't want to be dealing with an ECV on my own on the buses (probably a bit silly but that was how I felt!). My pace with the crutches and lightly-boot-tipping walk was pretty much a regular/quick walking pace. I will say that I was on European elbow crutches which meant no underarm strain - you just need some very strong arms and good padding for your hands. If your nephew could get his hands on elbow crutches he might find them nice to use.
I stayed onsite on my own but met up with family at different times during the week. It was very busy in WDW but I found the crutches to be no trouble in the crowds (again elbow crutches as slightly different - they tend to be used quite straight and not sticking out at the sides). Yes, they were a little tiring by the end of the day, but we were happily doing 12 hours days so everyone was tired by then
If you are going to Universal he will be treated like a king! A lot of their queues have stairs in them so we were directed to different queues by the staff. Very helpful. That was without any disability card though I think they may have one at guest services there - we didn't think to ask.
While WDW was fun on crutches, airports were not. So if you are flying call your airline and they will arrange a wheelchair for both airports - it makes it much quicker and easier. I could take the crutches onboard but mostly handled bathroom trips etc by putting on the walking boot and tipping along holding onto seats
Meals-wise, the times I was eating solo I asked if a cast member could drop my food down to my table at counter service places and they were very obliging.
I found the Florida heat caused my ankle to swell but the heat and humidity also helped my hands/arms to cope with the extra walking very well (dunno why, the heat just soaked away any aches!). Get your nephew to put the foot up whenever he sits down even if he is in an ECV, to help with swelling
So yes, I would say it is very doable (and very enjoyable) to do WDW on crutches if the conditions are right, depending on the specifics of the injury, strength etc.
Whatever you do, have a splendid trip and take lots of photos - it's not everyday one ends up in WDW on crutches!!!