Help Wobbles get medical care

I'm calling him Wobble because he can only use 3 of his legs. We started seeing Wobble around the neighborhood about 4 months ago and noticed the terrible wound to his leg. Unfortunately, it hasn't gotten better. Wobble is a neighborhood stray cat and he's decided that we belong to him now. 

We feed him and he's grown to trust us. He actually won't eat his food until we give him some head rubs and pet him. We want to get him the help he needs to get better. After talking to a local vet today, it sounds like amputation is the only way that will happen due to the severity of the wound and the length of time it has been an open wound. 

As much as we want to help Wobble get better, we don't have $1,500 to spend on him, which is about what it will cost for proper medical care and healing. So, we are asking for your help to help him heal.