Save My Best Friend

My dog is my best friend and my inspiration, he was an abused, stray, shelter dog and still learned to love, he lost the use of his leg and still loved to run.
I rescued him from a shelter about a year and a half ago. He was, abused and starved. He is only 3 and a half years old. He has come along way since I rescued him. Just recently he was hit by a truck. Thankfully nothing was broken, but he has severe nerve damagage. Because of the nerve damage it is considered a dead leg. He drags it along everywhere he goes, and chews on it. It is now become a paw of infection. There was nothing I could do to prevent this. He is his same happy, loving dog, just with three working legs. A few days ago we took him to the vet to see what we could do about it. Our only options are to put him down or amputate. Amputation is the only option to keep my dog healthy and alive but the cost is around $1000, what I'm asking is for anyone to donate anything they can to save my dog's life.

Extra info:
His name is King, he is roughly 3 and a half years old, he is a boxer pitbull mix and he loves life so please help him keep his life.