Depending on your location, you may have the option of a travelling vet that would be willing to come to you. If so then the tricky part is actually getting the cat into a controlled environment.
If you have a garage or are ok with having the cat in your home (assuming you do not have other pets that would complicate matters), then try and win the cat's trust. Go out and simply speak to it, try to give it food, etc. I have a cat that used to be a stray that all I did was go out with food which I put down and sat next to. It would come and eat and I would just talk to it. A couple of day's later he was sitting in my lap. Once you have the cat's trust you can have him follow you to the safe area (again garage, etc.) so that you can coordinate having the vet visit.
Be careful with trying to rush things and picking up the cat. He could hurt you if just trying to get away, and poor thing may not be exactly healthy.
I hope you are able to help it. Good luck.