Sounds like a bump he has had that has gone a little bit infected.
My daughter who is a vet wrote an article about just this type of injury for a Koi web site, but the treatment will be the same as treating your Carp.
It is an easy step by step procedure
Go here to view the web page. Koi Ulcer & Wound Treatment Tutorial
An ulcerated injury is relatively hard to treat under normal circumstances because the water washes away the medication which hinders the normal healing.
This treatment acts as a barrier to hold in the medications and certainly speeds up the healing process.
I hope this helps, it takes a bit of nerves to do the treatment as you have to handle the fish.
Hope this helps