I have never seen a snake swell like that prior to a shed. They sometimes appear slightly swollen before shedding, but as I said before, it is usually barely noticable. And it isn't a localized swelling like you have here. When a snake is about to shed, they can sometimes appear slightly puffy around the head/neck area as the fluids build up under that first layer of skin. But not like this.

It is possible that your snake banged his nose against the glass/side of his enclosure and this caused some swelling. I hope there is a simple reason and solution. But there could be an underlaying abscess, and abscesed tooth, fluids building up in the nasal passages, or some other cause that needs attention. You obviously love your snakes. I would not wait and see, I'd make SURE everything is alright and take him to a vet. Take that picture with you since the swelling has apparently gone down some now so the vet can see what it was like.