Thank you for taking care of this little one. I know how bad it feels to feel helpless in situations like this.
You are doing the right thing with the iodine. Povidone iodine is much better than tincture of iodine. If you can get a hold of a triple ingredient antibiotic cream for cats/dogs with Bacitracin, neomycin, and polymyxin that would be even better. Erythromycin is good as well. Avoid human antibiotics.
I assume the wound is not bleeding? If the cat is not bleeding, as long as you can keep the wound clean with the iodine, the wound should heal and the cat will be fine. Often times cats survive wounds like this without human intervention. If you can get a damp cloth and gently clean any blood or dirt that is around the wound gently, you can help prevent infection as well.
Watch for signs of infection: redness, swelling, weakness, lethargy, and see if the wound smells bad or is draining puss. If the wound becomes infected then the cat will need some antibiotics.
Since you are treating its wound, it sounds like the cat lets you handle it? If so you could quickly pick him up and drop him into a cardboard box or plastic bin. Have another person standing by to close the box when you drop him in. Wear some thick gloves if possible and make sure your forearms are covered with several layers of clothing. Once in the box you can get him to the vet.
I once had to catch a badly injured feral cat with a fishing net - the smaller ones with a hoop on the end of a short pole. If you have access to one you could capture him that way if you had to.
Let us know how the cat is doing and we can continue to advise.