If you've never treated/dealt with mouth rot your best bet would be to have a vet do the initial clean up and show you how to treat your snake. These infections are usually secondary to something else that is going on and it may need to be treated with a course of antibiotics to be able to clear this up completely in addition to debriding/treating the mouth.

I do not think your snake is going to want to eat while his mouth is this bad, the teeth/jaw can be affected when the "rot" gets as bad as in your picture.