Is he making any noises while breathing, like wheezing, crackling, or coughing? Rarely do low nighttime temps cause an URI, they actually want their nighttime temps to be considerably cooler than their daytime temps, as well as having it be pitch black, which is why you should never use any type of night light as any type of light disrupts their sleep greatly. The desert where they live in the wild gets very cool at night, and as long as his enclosure doesn't drop below 65 degrees you need no nighttime heat source at all, and your house would have to be kept at 60 degrees or lower for his enclosure to drop below 65 degrees, so chances are his nighttime temps are fine. A lot of people worry that they get too cold at night so they go out and buy a night bulb or a Ceramic Heat Emitter(CHE), which by the way is what you would need if his nighttime temps do drop below 65 degrees, and CHE, because they emit no light, just heat. So most people bump their nighttime temps into the 70's or 80's and the dragon is then very uncomfortable and cannot sleep due to the light or the temps being too high.
Usually if they get an URI due to being too cold it's because they were wet and became chilled from the dampness. If your dragon isn't wheezing, crackling, or coughing, or if you don't see a large amount of excess saliva in his mouth, he most likely is fine. They all have a little mucous in their mouths, so just seeing a small string of saliva when he opens his mouth up is not a sign of an URI. Neither is him opening his mouth up while basking, that is completely normal and he should be doing that, that is how they regulate their body temperature while basking. It does't mean they are sick, it doesn't mean that they are too hot, it's just them regulating their body temperature and it's normal.
It doesn't at all sound like your dragon is sick or has an upper respiratory infection to me at all. He does not need to be taken to the vet based on what you're saying, they will only put him on a broad-spectrum antibiotic on a "Just in Case" basis, which is unnecessary, dangerous, and will only cause him health problems he currently doesn't have. Just watch him, and if you hear him wheezing while he breaths, if you hear any crackling sounds when he breaths, if he starts coughing, or if he becomes noticeably lethargic, sleeping all day long and not moving at all, and he stops eating consistently for more than a day or two, then he should go to the vet. But based on what you're saying your dragon is perfectly fine.
It's very good that you love your dragon and you are concerned about his health and are willing to get him to the vet whenever he needs to go. That being said, what you need to remember is that most vets will not run a lot of diagnostic tests, if any at all, like cultures or x-rays or blood work, and all they end up doing is saying "Well, we can put him on a broad-spectrum antibiotic, like Baytril, since you're worried about him, "just in case" he is sick". This is very common, it's becoming more and more common, and the end result of putting a beardie on unnecessary medication without positively diagnosing a problem to begin with, is that they lose their appetite and stop eating, they lose weight, they become extremely lethargic, and some even die just from being on the unnecessary medications for long periods of time. It's very stressful for a bearded dragon to be taken to the vet and be put on oral medications, so unless you're certain that there is a real problem, it's best to just wait, watch him, and if he does display any lack of appetite, real lethargy, or any breathing sounds or issues, then take him to the vet.
