I can walk up to about a flat mile using two hiking sticks; these are much more maneuverable in crowds than crutches or my other mobility device, a rollator (a walker with a seat). A rollator is probably too bulky for you to take to Norway if your son does not recover in time, but two hiking sticks could easily be taken along. Or perhaps you could rent appropriate equipment when you arrive?
Will your trip involve a lot of stairs? With railings??? Sometimes with knee problems going up the stairs is fine and down is difficult. If you are going hiking, are trails groomed or rough?
Note on later edit: The above devices are not useful if he is not allowed to bear weight on the knee. A knee roller scooter would be useful for non-weightbearing injuries other than the knee. Ask the PT what types of assistive devices might be useful in your son's situation. Or perhaps consider postponing your trip.
2006 Tiger CX 4x4, 8.1 L gas V-8, Allison 6-speed