I wouldn't give up yet if the vet has set the leg properly he will still be fine he may have a slight limp but he will adjust to it have a look how quick a 3 legged dog can go once they master the handicap.
Using a shotgun for a fox you really need a choke to keep the shot together my trick is when I am loading shells for hogs or foxes is to pour hot wax in with the buckshot it keeps the shot together longer.a rifle much better choice for foxes.