If it smells then theres a VERY VERY VERY good chance gangrene has already set in. He needs to be on VERY STRONG antibiotics. Unfortunatly once the smell starts not much can be done without antibiotics. Most likey he will need to have some of the skin trimmed as the tissue is dying.

I am sorry I couldn't be of more help but he needs to see a vet. Once it starts to smell it is a dire emergancy!

Do you or your family have a credit card?
Or if you are a reputable client your vet should let you charge it.

One of the stray cats I found had to put to sleep because the gangrene was so bad and it was only about a 1.5 inch cut.