I need help!

I know this outside cat the shows up at my house, he basically grew up with us. Today I found him limping and he has a deep cut on his left front paw, either a bite or a BB pellet shot, there is another cut on the other side of his paw that might prove that he was shot but I'm not certain. so far I cleaned the wound, let him stay in the garage with food, water and a makeshift bed. I took him to the closest animal hospital and they said to make an appointment and that it would cost in the hundreds, my dad is not willing to pay that. what could I do to help the poor guy, his brother keeps trying to get close to him and clean him and the poor guy attacks him. What should I do?