Sounds like a burn, snakes really shouldn't have access to anything hot as they don't have the sense to keep away from it. If you can't find any more mites, the chances are it wasn't a mite, what you are describing sounds too big anyway. I would get something to cover the bulb, like a mesh cage to screw into the viv. Better still just take it out and achieve the temps with a UTH, none of my snakes have bulbs anymore ceramic heaters and UTH's are easier for me. Either way it isn't a big deal, the wound may be why he is going into shed, they are resilient creatures.