I had a gallon of milk in a paper bag in the back seat of my car that I forgot was there. When I remembered it was in there two days later and took it out, the heat had busted the jug, and the milk was gone, all down into my perforated leather seats.
Could some of the milk have run down behind and under the seat? You might need to pull the backseat out and clean under there.
There ia a product by Don Aslett called X-O odor neutralizer. It works wonders and is used for sour milk stains all the time. I have used it on cat odors You can find it at QVC.com, look It up as X-O concentrate, or by Don Aslett products. You will have to allow it to sit once sprayed on the leather to get out the odor that penetrated it pores and seams lines. After you clean the seats, do allow them plenty of fresh air even if you have to use a portable fan in the car seat to refresh and dry it. Then condition the leather to renew the leather finish. It will BE successful, and let us know how it worked out for you. I use X-O all the time.
Things that remove smells are baking soda and coffee grounds -- but not together. You could try washing the area with baking soda in water. Put a bucket of coffee grounds in the back seat and let it sit for a couple of days to see if it helps. I think with this powerful a smell you might have to go with some sort of commercial cleaner. This is not just a sour milk smell, but a rotten milk smell. And you have to contend with the leather -- so you don't want to use too strong a commercial cleaner, unless it is specially for leather.
This might be the time to contact a commercial car cleaning place and see if they can guarantee that they can get the smell out and not ruin the seat.
Another thought I just had -- can you remove the seat entirely from the car? The milk has obviously soaked well into the seat, and all the padding and such is soaked with this rotten milk. You have to be able to get at that with whatever you use, and just washing the surface is not going to do the trick.
Good luck with this. I think you are going to have to go to a professional cleaner.