Pretty little butterfly.

Looks like there has been some progession in the sore from the first picture to the second. I'd say it was injured in some way, and a bacterial infection has set in.

I would suggest starting treatment with something like Koizyme. You should be able to find it at a local pond store, but it can also be ordered online.

Hopefully others on the forum will have some addition suggestions.

I had the same sore on one of my Koi, I believe it was some water issues at the start of the season. I used Melafix, it's an anti bacterial wash. I added it to my pond, no isolation, I felt that would be too much stress on koi. It has since started healing nicely, I also had 2 koi with "Dotsy", they had that pine cone look.... Melafix brought them completely back to normal. I thought they were goners after reading that once they hit that "pine cone" stage surviving was slim.
Good luck,

http://www.pondcare.com/Products/Product.aspx?ProductID=10

how is your water kh.the kh keeps the ph steady that puts less stress on the fish.baking soda will raise the kh and is very cheap to use.

I bought some Melafix and put it in the tank last night. I hope that does the trick.
My ph is 8.2, and it rarely varies from that. If it does change, I might get an occasional 8.4 but I have never seen it below 8. I don't have a test that specifically tests kh, but I know we have very hard water here.
I'm not clear on understanding the relationship between kh and ph though. I have a mental block when it comes to chemistry (and physics). The only things left between me and a degree in Zoology are Chemistry and Physics classes. I don't think I will ever graduate!

To be honest, that looks like hole in the head disease. The causes are not entirely known although it may be a combination of stress and parasites. I have had koi with the condition and treatment can be frustrating and not always successful.

Here is a link that might be useful: Hole in the head disease

Heather, I'm sorry your fish has a sore. Hope he gets better soon. He is really lovely.