I'm sorry to hear all the struggles this poor guy is having.
You are such a saint to take him in. Now, I know that amputating a leg seems so cruel and a last resort, but I watch Animal Planet all the time, and the vets say it's not as bad as we think it is for the pups. They get along very well without a limb and just adjust. He will still be a happy boy with 3 legs. Whatever you decide to do, good luck! I know it would be a very hard decision for me to make too. Hugs to you.