Heck that don't look so bad to me.
I found one this spring that had a much nastier wound that had just about finished healing. My opinion, keep him warm dry gooped up with a little neosporin and offer him a worm every now and then and he will be fine. This is coming from someone that knows nothing about snake care, but knows how incredibly tough wild creatures can be. OH! Raise your lawnmower blade a couple of notches.
