The open mouth breathing "could" have something to do with your snakes current shed cycle if some of the old skin is interferring with his ability to breathe normally.
However, it still bears keeping an eye on. Allow him to finish shedding and see if he still has any symptoms.
In the meantime keep an eye out for any of the other symptoms that may mean he has a potential RI:
- gurgling, whistling, or crackling when he breathes
- sometimes, "coughing" can be heard, technically, your snake cannot cough, but when phlegm obstructs his airway and is cleared through a deep or forceful breath it can sound a lot like a cough
- excessive salivation...your snake will drool
- "cheesy" looking substance in the mouth
- when they drool or have a discharge, many snakes will smear this substance on the walls of their tub, enclosure or tank (whichever you use)...usually when this has dried it looks like a thin layer of egg whites was smeared...be on the look out for this if you do not directly observe either drooling or discharge...this will tell you it's going on even if you don't actually see it since sometimes the discharge or drool can be in small, almost unnoticable quantities
- lethargy - less active than usual, loss in muscle tone, general weakness, slow moving
-"snot" bubbles coming from his nostrils when he breathes (just like a little kid with a cold)
RI is serious and isn't fun to deal with. Anyone who says they've never dealt with it is either full of it or hasn't been keeping snakes for very long. The first time I dealt with RI I was told to lower the humidity and increase the temperatures (slightly)...now, I was in the same boat as you because the snake I was dealing with was also entering a shed cycle so lowering the humidity really didn't seem like a smart idea.
If it is RI, the question isn't whether your snake will survive one bad shed...it's whether it will survive one bad respiratory infection. I was dealing with a 200 gram baby...she is now a very robust adult...went through a bad shed but the RI cleared up very quickly. The only other time I had a snake with an RI, I had left for the weekend only to find out when I came home that the power had been knocked out shortly after I left due to a bad storm...all of my heat failed. I came home to very cold snakes. Luckily, only one got sick...but the infection spread fast and (despite antibiotics and a very dedicated vet) she passed away.
I know I'm most likely going over board on this and I apologize if I'm coming across as being condescending, I suppose I would rather offending someone (unintentionally, of course) by offering too much information than leave them hanging by not giving enough.
Keep an eye on him. Let him shed out, if the symptoms are still there, get worse or any of the above mentioned symptoms appear (no matter how benign they seem) set up an appointment with a vet as soon as possible...RI is fairly easily treatable in most cases IF it is caught soon enough, but can be fatal in a hurry otherwise.