Maybe he might be starting to shed, and is attempting to loosen the skin he will be shedding. Mine did that when he was about to shed, he'd open his mouth and yawn and gape so the skin would come loose.
But, if he seems to be staying still with his head pointed up, mouth open, bubbling out of his mouth, it might be an upper respiratory infection. Look for bubbles on his mouth, listen to see if he makes any noise when he breathes. It might be a URI then take him to the vet.