I have seen amazing recoveries after physical injuries. The fact that he already survived for two weeks gives me hope, but the lack of nutrition might be a problem. The separate(probably small) container in the pond might add unnecessary stress, but it needs food to get his strength back. The ideal would probably be to place him in a heated hospital facility of at least 1000 liters, with salt @ 3kg/1000 liters to calm him and protect him from secondary parasite/bacterial infections because of a low immunity due to stress, and see what happens.
The possibility of survival is probably 50/50.