Angie...zodiacqueen is right. You need to let someone experienced with handling feral cats take care of this situation. You also need to be aware that the threat of rabies is very real with feral cats and you should take any bite or scratch he gives you seriously. Odds are that he probably doesn't have rabies, but it's a big risk to take.
Have your local AC or humane society place a humane trap. They can catch him and then evaluate the wound.
Hi, you should call your local animal control, ASPCA, or humane society and ask to borrow a trap to catch him and get him in to see a vet. I'm positive that one of these organizations will help you once you explain this situation to them.
I honestly believe that the wound could get infected and it sounds like it's very deep. I wouldn't take a chance w/ it. It needs to be treated before it becomes very infected.
Good luck, and please come back w/ an update!