There's a sick stray cat I want to help-?

He is older and has been sick for over a year, and has been worse than he is now. He was black, but his fur is slowly changing to a reddish-brown color and it is thinning and falling out in places. His ears have small yellow-brown globs extending from the inside skin to the sides of his face. He's lost weight. Patches of his fur are spiked up with a hard mucous. I can't take him to the vet, but he does eat the food I leave for him. Is there anything I can do?