Is he holding his nose in the air at all? Have you noticed if he’s making any popping or other noises while breathing? Is he shedding?
Chameleons do yawn and I often see one of mine do it…or she’s getting ready to try to bite me (another thing she often does). When shedding, they will gape and do all sorts of things to loosen up the shed skin. Hearing any noises, holding his nose in the air (star gazing), thick ropy saliva are all signs of respiratory infection that need a vet visit asap.