If you can take him to the vet, you should start there. Proper vet care is always the first step to ensure a smooth and uncomplicated healing. Skipping the vet, even on basic situations, can lead to more expensive complications down the road.
If you cannot get to a vet, if possible you need to keep the cat indoors until the wound heals to minimize risk of infection. Right now the main goal (outside of a vet) would be to keep the wound clean and your cat well fed while healing so it's body has energy to heal. Depending on how deep the wound is, you either want to dip the area in a clean bowl of boiled (not boiling, but boiled and cooled) water with a few drops of either hydrogen peroxide or betadine in it or wipe the area down with the same mix. Because of where it is, it really isn't safe to apply any human topical ointments because your cat will just lick it off. You will want to repeat the cleaning (although don't add the hydrogen peroxide after the First one, that's just to help remove any debris the first time more and it can slow healing so o ly use hydrogen peroxide in the first washing) as often as your cat will tolerate.
If the wound is deep you really need to find a way to see a vet as deep wounds have a greater risk of infection and if it is really deep the infection can get in the bone or spread rapidly. It's hard to tell depth in the photo. Also, if you see any sign of infection, spreading redness or puss, you need to see a vet to control the infection. It does look red right now but it's hard to tell if that skin coloring or blood smear or what but if the area is warmer to the touch than touching between toes on a different paw, that is also a sign of infection and you need to see a vet.
In summary, see a vet. If you can't see a vet, make sure your cat stays indoors and keep the area clean. If it is deep, or you see signs of infection setting in, seeing a vet must become a priority.