Hi Fraoch

I wouldn't be too concerned about treating the ankle/knee - not much options there at present eh!

However you could consider what to do to maintain/increase aerobic fitness and muscle strength elsewhere. I take it he is in Plaster POP so you can't put him in the pool. that is a pity because the pool opens up options. If it is fibreglass or other type of splinting you can often get away with the pool.

What about setting up a circuit training involving working the other leg (if you have a reclining ergometer and fasten his foot to the pedal while keeping the , working the arms (upper arm ergometer if you have it) or light weights - hi reps

You could use Progressive resistance training equipment for strengthening - just have to choose exs that don't include the affected leg in any way. You should be going for high intensity (low reps) here.

don't forget to include hip exs for the affected leg.