definitely an abscess of some sort, most likely stemming from somewhere in the upper respiratory area. Could you fill out the how to ask for help form?
That looks like a serious infection in one of the nasal cavities. I'm not a vet but any other would tell you the same - get your chameleon to a vet and have that drained and treated ASAP.
Please don't hesitate to visit your reptile vet. Noone can tell only from a picture whether this is an abscess or an bone infection or...Eventually your vet wants to take an x-ray to assure there's no infection concerning the jaw bone yet, cause chameleons unfortunately tend to spreading infections around (and inside) their mouth.
That doesn't mean he has no pain or that it isn't that fatal. Chameleons always try to hide every little sign of weakness. They're wild animals, a weakened chameleon is an easy prey for predators in nature. They behave same way at home in their cages. Just take a look at these pictures from an adult Furcifer pardalis with stomatitis. His whole head was full of pus, hardly a bone still was intact - but he ate completely normally until two days before euthanasia. I don't want to fear you, but pics should show why it's better to visit your reptile vet before the chameleon can't hide his sickness anymore, especially with problems in head region.
He drained all the puss outta the abscess & then applied something called Xenodine to the wound, & gave me the bottle & told me to apply the stuff to the wound daily for like 3 weeks until he heals.