I assume one of those things that rolls that you rest your leg on is out, too?
What about a manual wheelchair, if he's otherwise fairly fit?
Presumably he'll need to use crutches some of the time - if he can get forearm (aka Lofstrand, Canadian, or elbow) crutches they are vastly more efficient and comfortable than axillary crutches. If they are not supplied by the hospital or surgical center, they can be purchased for a under a hundred dollars.