Can you keep him inside? Give him water w/ Vitimans, a good quality food, some treats. Try giving physical therapy to the leg. Rub the skin and push it in and out for a few minutes. Getting the leg to function is lower on the scale, you want to stabilize his condition. Keep him somewhere warm but not too hot, wouldn't hurt to worm him and dust him with anti-mite / lice dust, either, if you have it on hand.
Best of luck, his system is strained so you want to be able to just let him rest and heal without having to monitor his heat too much or strain about parasites, etc.