That looks like an infection... Can't tell if that white stuff is just debris or the sort of cottage-cheese looking residue associated with stomatitis, but you should quarantine this guy and power clean everything he's been on (especially if other snakes are around) just to be safe. If this is actually stomatitis and not a regular mouth infection, I'm sure someone who has dealt with it before will be able to give you advice on how to handle that.

If it's getting worse quickly, you might want to treat with diluted hydrogen peroxide first (I find that H202 works well to initially 'nuke' really worrisome infections, but you don't want to use it for long as it can seriously damage the tissue). After that, I'd do twice-daily swabbings with diluted hibiclens (betadine works as well, but my snakes don't seem to mind hibiclens as much) and keep that up for awhile.
Also, you may want to change to using newspaper as substrate for the time being. Where the mouth can't seal 100%, dust and debris from most bedding will stick to it and make healing up that much harder.