Ouch, he did a great job on that one!!!
A friend of mine has a young horse that did a similar injury, but a little further forward right where the girth sits. It took a number of month for it to heal completely, but now is just a fine line with a small bump. It doesn't effect her in any way and she is out doing extremely well in dressage.
Best of luck, it will be a bugger of a wound to look after!