Either you or the vet needs to get a good look inside his mouth. You probably need help restraining him also, to do this effectively. Snakes are
adorable when they yawn but they never do it when we need them to. What kind of substrate is in his cage? It's possible that he has a bit of
substrate stuck in his teeth. It's also possible he is getting a mouth infection of some kind. An RI does NOT normally manifest with "stargazing",
by the way, & in the beginning the symptoms are mild...just like when we get a cold. If it actually IS an RI, it's better to treat sooner than later,
and bubbling mucus is also a later symptom. Lethargy is also when an RI is so far along that the snake is weak from difficulty breathing...do
NOT wait for that, please? (snakes cannot cough out the mucus...they can literally die from congestion)

As far as his most recent shed, are you SURE he got ALL the skin off? It often gets stuck on their lower jaw, & I'd imagine that some stuck skin
could make that side of his mouth feel tight, so that's another thing to look closely at before you run to the vet. (After a snake sheds, it's always
best to check that their eye caps, tail tip & jaw have no retained skin to cause problems.) It can be VERY hard to see retained skin on a snake's
chin: what you might do to make it easier (& that he'll probably enjoy) is to put him in a "critter cottage" with shallow water, soaking wet paper
towels, saturated moss or something similar) for about 2 hours...then check his chin gently with your fingers & even more carefully with your
fingernail to see if old skin comes off. I have been surprised more than once doing this with one of my corn snakes- & I hope that's all it is.