Hi Steve,
now that your son is out of plaster i would start simple mobilising exercises. foot up-down, rotate in/out and ankle circling. in another week progress to weight bearing stretches for the calf. isometric exerises could be started immediately. this involves working the foot muscles statically(ie pushing hard against something such as a wall that doesnt allow any movement) because there is no movement you cannot injure the ligaments further. after about 3 weeks begin balance training. wobble boards or simply wakling atightrope on the floor works well for this. as a rough guide he will probably be about 80% heaed at 3 months. this should be enough for light training with view to return to sport at 4 months post injury. hope this helps