Show pics of the enclosure and tell us what temps and humidty is and what the substrate is and type of lighting and heat. Also what are you using to read the temps?
Diet and how often do you give warm water soaks and does he have a water dish.
Also try to look on his mouth to see if something is stuck in there.
Be sure the heat is off at night to no lower then 60 degrees. During The day basking spot of 95-100 but the rest of the enclosure varied 75-80.
My first guess would be over heated second guess something in mouth/throat.
If it is a respiratory then follow what Yvonne said. My temps suggestion is for a healthy Russian.