Looks identical to what my old TB used to do. He moved quite close behind, and tended to get flares on the inside of his hinds very quickly despite regular trimming.
You want to be careful with these as they can calcify and effect the joint.
I suggest that you clean the wound up with anti-septic cream, put a good sized dollop of white ointment on a piece of gauze, bandage that on and then put a rubber ring around the fetlock. A bell boot won't do much as they only protect the hoof and coronet.
This the the ring that I'm talking about
: Brighton Saddlery - Boots & Bandages - Rubber Fetlock Ring -
You can get them off Ebay very cheaply, I think they are also called sausage boots?
My gelding had this issue before I purchased him, and had 3 separate wounds in the same area. By keeping them soft and clean with white ointment and bandaging, and protected with the ring boot, they cleared up within a few weeks - and these had already started to calcify so a fresh one like yours shouldn't take long at all.
To prevent them, paddock him with the rings if you can, and talk to your farrier about trimming/shoeing in a way that will make your horse track slightly wider behind rather than coming in narrow and clipping himself.
I wouldn't bother with a vet for this one - it's a pretty easy fix.