SAB Punctured his Lower Jowl and will not Heal

I have a young SAB that I has started work in Protection Training.  About a month ago he punctured/fanged his lower jowl during our training.  I had left it alone (making sure it did not get infected) in hopes that it would heal on its own.  But unfortunately his lower K9 tooth kept/keeps getting hooked in the small punctured hole and his upper K9 tooth tends to hender the healing process also because it sits right there when his mouth is closed.  About a week ago I took him to the vets where they made the hole a bit bigger and then stitched it up.  Well less than a 7 days later the stitches are gone and the hole (which is not noticable until it gets hooked on his tooth) is back.  I know this has to be painful and we have stopped his bitework for now.

Have any of you experienced this with your Mastiffs?  Do I need to have it stitched again or perhaps with no biting activity will it slowly heal on its own?   Any advice/tips would be grealty appreciated.